diff --git a/packages/wct-mocha/src/childrunner.ts b/packages/wct-mocha/src/childrunner.ts
index 1dc603f32..589f6c90a 100644
--- a/packages/wct-mocha/src/childrunner.ts
+++ b/packages/wct-mocha/src/childrunner.ts
@@ -32,9 +32,9 @@ export default class ChildRunner {
   private container?: HTMLDivElement;
   private eventListenersToRemoveOnClean: EventListenerDescriptor[] = [];
   private iframe?: HTMLIFrameElement;
-  private onRunComplete: (error?: {}) => void;
-  private share: SharedState;
-  private state: 'initializing'|'loading'|'complete';
+  private onRunComplete: ((error?: {}) => void)|null = null;
+  share: SharedState|null = null;
+  state: 'initializing'|'loading'|'complete';
   private timeoutId?: number;
   private url: string;
 
@@ -88,7 +88,7 @@ export default class ChildRunner {
    * @return {ChildRunner} The `ChildRunner` that was registered for this
    * window.
    */
-  static current(): ChildRunner {
+  static current(): ChildRunner|null {
     return ChildRunner.get(window);
   }
 
@@ -97,7 +97,7 @@ export default class ChildRunner {
    * @param {boolean} traversal Whether this is a traversal from a child window.
    * @return {ChildRunner} The `ChildRunner` that was registered for `target`.
    */
-  static get(target: Window, traversal?: boolean): ChildRunner {
+  static get(target: Window, traversal?: boolean): ChildRunner|null {
     const childRunner = ChildRunner.byUrl[target.location.href];
     if (childRunner) {
       return childRunner;
@@ -154,7 +154,7 @@ export default class ChildRunner {
         () => this.loaded(new Error('Failed to load document ' + this.url)),
         iframe);
     this.addEventListener(
-        'DOMContentLoaded', () => this.loaded(), iframe.contentWindow);
+        'DOMContentLoaded', () => this.loaded(), iframe.contentWindow!);
   }
 
   /**
@@ -165,15 +165,15 @@ export default class ChildRunner {
   loaded(error?: {}) {
     util.debug('ChildRunner#loaded', this.url, error);
 
-    if (this.iframe.contentWindow == null && error) {
+    if (!this.iframe || this.iframe.contentWindow == null && error) {
       this.signalRunComplete(error);
       this.done();
       return;
     }
 
     // Not all targets have WCT loaded (compatiblity mode)
-    if (this.iframe.contentWindow.WCT) {
-      this.share = this.iframe.contentWindow.WCT.share;
+    if (this.iframe.contentWindow!.WCT) {
+      this.share = this.iframe.contentWindow!.WCT.share;
     }
 
     if (error) {
@@ -215,7 +215,7 @@ export default class ChildRunner {
       setTimeout(() => {
         this.removeAllEventListeners();
 
-        this.container.removeChild(this.iframe as Node);
+        this.container!.removeChild(this.iframe as Node);
         this.iframe = undefined;
         this.share = null;
       }, 0);
diff --git a/packages/wct-mocha/src/clisocket.ts b/packages/wct-mocha/src/clisocket.ts
index 6a6210d23..d88d1e84b 100644
--- a/packages/wct-mocha/src/clisocket.ts
+++ b/packages/wct-mocha/src/clisocket.ts
@@ -98,7 +98,7 @@ export default class CLISocket {
    *
    * @param {function(*, CLISocket)} done Node-style callback.
    */
-  static init(done: (error?: {}, socket?: CLISocket) => void) {
+  static init(done: (error?: {}|null|undefined, socket?: CLISocket) => void) {
     const browserId = util.getParam('cli_browser_id');
     if (!browserId) {
       return done();
@@ -108,7 +108,7 @@ export default class CLISocket {
       return done();
     }
 
-    util.loadScript(SOCKETIO_LIBRARY, (error: {}) => {
+    util.loadScript(SOCKETIO_LIBRARY, (error: {}|undefined|null) => {
       if (error) {
         return done(error);
       }
@@ -139,7 +139,7 @@ export default class CLISocket {
  * @return {!Array.<string>} The titles of the runnable and its parents.
  */
 function getTitles(runnable: Mocha.IRunnable) {
-  const titles = [];
+  const titles: string[] = [];
   while (runnable && !runnable.root && runnable.title) {
     titles.unshift(runnable.title);
     runnable = runnable.parent as {} as Mocha.IRunnable;
diff --git a/packages/wct-mocha/src/config.ts b/packages/wct-mocha/src/config.ts
index e867e8a96..0667fc969 100644
--- a/packages/wct-mocha/src/config.ts
+++ b/packages/wct-mocha/src/config.ts
@@ -76,7 +76,7 @@ export function setup(options: Partial<Config>) {
     const wctMochaJsRoot = util.scriptPrefix('wct-mocha.js');
     const browserJsRoot = util.scriptPrefix('browser.js');
     const scriptName = wctMochaJsRoot ? 'wct-mocha.js' : 'browser.js';
-    const root = wctMochaJsRoot || browserJsRoot;
+    const root = (wctMochaJsRoot || browserJsRoot)!;
     _config.root = util.basePath(root.substr(0, root.length - 1));
     if (!_config.root) {
       throw new Error(
@@ -93,11 +93,13 @@ export function get<K extends keyof Config>(key: K): Config[K] {
   return _config[key];
 }
 
-export function deepMerge(target: Partial<Config>, source: Partial<Config>) {
-  Object.keys(source).forEach((key) => {
-    if (target[key] !== null && typeof target[key] === 'object' &&
-        !Array.isArray(target[key])) {
-      deepMerge(target[key], source[key]);
+export function deepMerge<V extends {}>(target: V, source: V) {
+  Object.keys(source).forEach((untypedKey) => {
+    const key = untypedKey as keyof typeof source;
+    const targetValue = target[key];
+    if (targetValue != null && typeof targetValue === 'object' &&
+        !Array.isArray(targetValue)) {
+      deepMerge(targetValue, source[key]);
     } else {
       target[key] = source[key];
     }
diff --git a/packages/wct-mocha/src/environment.ts b/packages/wct-mocha/src/environment.ts
index 757e29d03..8a6a4d029 100644
--- a/packages/wct-mocha/src/environment.ts
+++ b/packages/wct-mocha/src/environment.ts
@@ -30,7 +30,7 @@ export function loadSync() {
     scripts.push(a11ySuiteScriptPath);
   }
   scripts.forEach((path) => {
-    const url = util.expandUrl(path, config.get('root'));
+    const url = util.expandUrl(path, config.get('root')!);
     util.debug('Loading environment script:', url);
     // Synchronous load.
     document.write(`<script src="${encodeURI(url)}"></script>`);
@@ -38,7 +38,7 @@ export function loadSync() {
   util.debug('Environment scripts loaded');
   const imports = config.get('environmentImports');
   imports.forEach((path) => {
-    const url = util.expandUrl(path, config.get('root'));
+    const url = util.expandUrl(path, config.get('root')!);
     util.debug('Loading environment import:', url);
     // Synchronous load.
     document.write(`<link rel="import" href="${encodeURI(url)}">`);
diff --git a/packages/wct-mocha/src/environment/helpers.ts b/packages/wct-mocha/src/environment/helpers.ts
index 6962b87f4..be9c7a13f 100644
--- a/packages/wct-mocha/src/environment/helpers.ts
+++ b/packages/wct-mocha/src/environment/helpers.ts
@@ -132,7 +132,7 @@ export function flush(callback: () => void) {
   } else if (window.WebComponents && window.WebComponents.flush) {
     scope = window.WebComponents;
   }
-  if (scope) {
+  if (scope && scope.flush) {
     scope.flush();
   }
 
diff --git a/packages/wct-mocha/src/index.ts b/packages/wct-mocha/src/index.ts
index d632fd8a3..65245fab7 100644
--- a/packages/wct-mocha/src/index.ts
+++ b/packages/wct-mocha/src/index.ts
@@ -58,7 +58,7 @@ export function initialize(initConfig?: config.Config) {
     // Until then, we get to rely on it to expose parent runners to their
     // children.
     _ChildRunner: ChildRunner,
-    _reporter: undefined as MultiReporter,  // assigned below
+    _reporter: undefined! as MultiReporter,  // assigned below
     _config: config._config,
 
     // Public API
diff --git a/packages/wct-mocha/src/mocha/extend.ts b/packages/wct-mocha/src/mocha/extend.ts
index d775cf15c..f25ce11b5 100644
--- a/packages/wct-mocha/src/mocha/extend.ts
+++ b/packages/wct-mocha/src/mocha/extend.ts
@@ -28,7 +28,10 @@ export function extendInterfaces(
   interfaceExtensions.push(() => {
     const Mocha = window.Mocha;
     // For all Mocha interfaces (probably just TDD and BDD):
-    Object.keys(Mocha.interfaces).forEach((interfaceName: 'tdd'|'bdd') => {
+    Object.keys(Mocha.interfaces).forEach((interfaceName: string) => {
+      if (interfaceName !== 'tdd' && interfaceName !== 'bdd') {
+        return;
+      }
       // This is the original callback that defines the interface (TDD or
       // BDD):
       const originalInterface = Mocha.interfaces[interfaceName];
@@ -44,16 +47,22 @@ export function extendInterfaces(
         originalInterface.apply(this, arguments);
         // Register a listener so that we can further extend the base
         // interface:
-        suite.on('pre-require', (context: {}, _file: string, _mocha: {}) => {
-          // Capture a bound reference to the teardown function as a
-          // convenience:
-          const teardown = context[teardownProperty].bind(context);
-          // Add our new helper to the testing context. The helper is
-          // generated by a factory method that receives the context,
-          // the teardown function and the interface name and returns
-          // the new method to be added to that context:
-          context[helperName] = helperFactory(context, teardown, interfaceName);
-        });
+        suite.on(
+            'pre-require',
+            (context: {teardown: Function, afterEach: Function},
+             _file: string,
+             _mocha: {}) => {
+              // Capture a bound reference to the teardown function as a
+              // convenience:
+              const teardown = context[teardownProperty].bind(context);
+              // Add our new helper to the testing context. The helper is
+              // generated by a factory method that receives the context,
+              // the teardown function and the interface name and returns
+              // the new method to be added to that context:
+              // tslint:disable-next-line:no-any
+              (context as any)[helperName] =
+                  helperFactory(context, teardown, interfaceName);
+            });
       };
     });
   });
diff --git a/packages/wct-mocha/src/reporters.ts b/packages/wct-mocha/src/reporters.ts
index b9284507b..4cab60c34 100644
--- a/packages/wct-mocha/src/reporters.ts
+++ b/packages/wct-mocha/src/reporters.ts
@@ -24,10 +24,10 @@ export let jsSuites: Array<undefined> = [];
  * @return {!Array.<!Mocha.reporters.Base} The reporters that should be used.
  */
 export function determineReporters(
-    socket: CLISocket, parent: MultiReporter): ReporterFactory[] {
+    socket: CLISocket, parent: MultiReporter|null): ReporterFactory[] {
   // Parents are greedy.
   if (parent) {
-    return [parent.childReporter(window.location)];
+    return [parent.childReporter()];
   }
 
   // Otherwise, we get to run wild without any parental supervision!
diff --git a/packages/wct-mocha/src/reporters/console.ts b/packages/wct-mocha/src/reporters/console.ts
index ae3c54d7e..9c56ac610 100644
--- a/packages/wct-mocha/src/reporters/console.ts
+++ b/packages/wct-mocha/src/reporters/console.ts
@@ -34,10 +34,17 @@ const CAN_STYLE_GROUP = userAgent.match('webkit');
 // Track the indent for faked `console.group`
 let logIndent = '';
 
+function getStyle(style?: keyof typeof STYLES): string {
+  if (style === undefined) {
+    return STYLES.plain;
+  }
+  return STYLES[style] || STYLES.plain;
+}
+
 function log(text: string, style?: keyof typeof STYLES) {
   text = text.split('\n').map((l) => logIndent + l).join('\n');
   if (CAN_STYLE_LOG) {
-    console.log('%c' + text, STYLES[style] || STYLES.plain);
+    console.log('%c' + text, getStyle(style));
   } else {
     console.log(text);
   }
@@ -45,7 +52,7 @@ function log(text: string, style?: keyof typeof STYLES) {
 
 function logGroup(text: string, style?: keyof typeof STYLES) {
   if (CAN_STYLE_GROUP) {
-    console.group('%c' + text, STYLES[style] || STYLES.plain);
+    console.group('%c' + text, getStyle(style));
   } else if (console.group) {
     console.group(text);
   } else {
diff --git a/packages/wct-mocha/src/reporters/html.ts b/packages/wct-mocha/src/reporters/html.ts
index 97df6595f..4b8652335 100644
--- a/packages/wct-mocha/src/reporters/html.ts
+++ b/packages/wct-mocha/src/reporters/html.ts
@@ -14,12 +14,12 @@
  *
  * @param {!Mocha.Runner} runner The runner that is being reported on.
  */
-export default function HTML(runner: Mocha.IRunner) {
+export default function HTML(this: Mocha.IRunner, runner: Mocha.IRunner) {
   const output = document.createElement('div');
   output.id = 'mocha';
   document.body.appendChild(output);
 
-  runner.on('suite', function(_test: {}) {
+  runner.on('suite', function(this: Mocha.IRunner, _test: {}) {
     this.total = runner.total;
   }.bind(this));
 
@@ -67,4 +67,4 @@ style.textContent = `
       color: #555 !important;
     }
 `;
-document.head.appendChild(style);
+document.head!.appendChild(style);
diff --git a/packages/wct-mocha/src/reporters/multi.ts b/packages/wct-mocha/src/reporters/multi.ts
index 5f20fe3b1..ad8c7c02e 100644
--- a/packages/wct-mocha/src/reporters/multi.ts
+++ b/packages/wct-mocha/src/reporters/multi.ts
@@ -48,16 +48,16 @@ export interface ReporterFactory {
 }
 
 interface ExtendedTest extends Mocha.ITest {
-  err: {};
+  err: {}|undefined;
 }
 
 /**
  * A Mocha-like reporter that combines the output of multiple Mocha suites.
  */
 export default class MultiReporter implements Reporter {
-  private readonly reporters: ReadonlyArray<Reporter>;
-  private readonly parent: MultiReporter|undefined;
-  private readonly basePath: string;
+  readonly reporters: ReadonlyArray<Reporter>;
+  readonly parent: MultiReporter|undefined|null;
+  readonly basePath: string;
   total: number;
   private currentRunner: null|Mocha.IRunner;
   /** Arguments that would be called on emit(). */
@@ -73,7 +73,7 @@ export default class MultiReporter implements Reporter {
    */
   constructor(
       numSuites: number, reporters: ReporterFactory[],
-      parent: MultiReporter|undefined) {
+      parent: MultiReporter|undefined|null) {
     this.reporters = reporters.map((reporter) => {
       return new reporter(this);
     });
@@ -92,12 +92,10 @@ export default class MultiReporter implements Reporter {
   }
 
   /**
-   * @param location The location this reporter represents.
    * @return A reporter-like "class" for each child suite
    *     that should be passed to `mocha.run`.
    */
-  childReporter(location: Location|string): ReporterFactory {
-    const name = this.suiteTitle(location);
+  childReporter(): ReporterFactory {
     // The reporter is used as a constructor, so we can't depend on `this` being
     // properly bound.
     const self = this;
diff --git a/packages/wct-mocha/src/reporters/title.ts b/packages/wct-mocha/src/reporters/title.ts
index f94f2f645..276f9294d 100644
--- a/packages/wct-mocha/src/reporters/title.ts
+++ b/packages/wct-mocha/src/reporters/title.ts
@@ -20,7 +20,6 @@ const ARC_WIDTH = 6;
  * @param {!Mocha.Runner} runner The runner that is being reported on.
  */
 export default class Title {
-  runner: Mocha.Runner;
   constructor(runner: Mocha.IRunner) {
     Mocha.reporters.Base.call(this, runner);
 
@@ -46,7 +45,7 @@ export default class Title {
   updateFavicon() {
     const canvas = document.createElement('canvas');
     canvas.height = canvas.width = 32;
-    const context = canvas.getContext('2d');
+    const context = canvas.getContext('2d')!;
 
     const passing = this.stats.passes;
     const pending = this.stats.pending;
@@ -61,9 +60,9 @@ export default class Title {
 
   /** Sets the current favicon by URL. */
   setFavicon(url: string) {
-    const current = document.head.querySelector('link[rel="icon"]');
+    const current = document.head!.querySelector('link[rel="icon"]');
     if (current) {
-      document.head.removeChild(current as Node);
+      document.head!.removeChild(current as Node);
     }
 
     const link = document.createElement('link');
@@ -71,7 +70,7 @@ export default class Title {
     link.type = 'image/x-icon';
     link.href = url;
     link.setAttribute('sizes', '32x32');
-    document.head.appendChild(link);
+    document.head!.appendChild(link);
   }
 }
 
diff --git a/packages/wct-mocha/src/stacktrace/formatting.ts b/packages/wct-mocha/src/stacktrace/formatting.ts
index 75b5838a4..bd09a3506 100644
--- a/packages/wct-mocha/src/stacktrace/formatting.ts
+++ b/packages/wct-mocha/src/stacktrace/formatting.ts
@@ -15,27 +15,26 @@ export type FormattingFunction = (text: string) => string;
 export interface FormattingOptions {
   showColumns?: boolean;
   // Methods are aligned up to this much padding.
-  maxMethodPadding?: number;
+  maxMethodPadding: number;
   // A string to prefix each line with.
-  indent?: string;
+  indent: string;
   // A string to show for stack lines that are missing a method.
-  methodPlaceholder?: string;
+  methodPlaceholder: string;
   // A list of Strings/RegExps that will be stripped from `location` values
   // on each line (via `String#replace`).
-  locationStrip?: (string|RegExp)[];
+  locationStrip: (string|RegExp)[];
   // A list of Strings/RegExps that indicate that a line is *not* important,
   // and should be styled as such.
-  unimportantLocation?: Array<string>;
+  unimportantLocation: Array<string>;
   // A filter function to completely remove lines
-  filter?: (line: ParsedLine) => boolean;
+  filter: (line: ParsedLine) => boolean;
   // styles are functions that take a string and return
   // that string when styled.
-  styles?: {
-    method?: FormattingFunction;
-    location?: FormattingFunction;
-    line?: FormattingFunction;
-    column?: FormattingFunction;
-    unimportant?: FormattingFunction;
+  styles: {
+    method: FormattingFunction; location: FormattingFunction;
+    line: FormattingFunction;
+    column: FormattingFunction;
+    unimportant: FormattingFunction;
   };
 }
 
@@ -56,14 +55,15 @@ export const defaults: FormattingOptions = {
 };
 
 export function pretty(
-    stackOrParsed: string|ParsedLine[], options?: FormattingOptions): string {
-  options = mergeDefaults(options || {}, defaults);
-  let lines =
+    stackOrParsed: string|Array<ParsedLine|null>,
+    maybeOptions?: Partial<FormattingOptions>): string {
+  const options = mergeDefaults(maybeOptions || {}, defaults);
+  const linesAndNulls =
       Array.isArray(stackOrParsed) ? stackOrParsed : parse(stackOrParsed);
-  lines = clean(lines, options);
+  const lines = clean(linesAndNulls, options);
 
   const padSize = methodPadding(lines, options);
-  const parts = lines.map((line: ParsedLine) => {
+  const parts = lines.map((line) => {
     const method = line.method || options.methodPlaceholder;
     const pad = options.indent + padding(padSize - method.length);
     const locationBits = [
@@ -84,10 +84,11 @@ export function pretty(
   return parts.join('\n');
 }
 
-function clean(lines: ParsedLine[], options: FormattingOptions): ParsedLine[] {
-  const result = [];
+function clean(
+    lines: Array<ParsedLine|null>, options: FormattingOptions): ParsedLine[] {
+  const result: ParsedLine[] = [];
   for (let i = 0, line; line = lines[i]; i++) {
-    if (options.filter(line)) {
+    if (options.filter && options.filter(line)) {
       continue;
     }
     line.location = cleanLocation(line.location, options);
@@ -104,24 +105,27 @@ function passthrough(text: string): string {
   return text;
 }
 
-function mergeDefaults(
-    options: FormattingOptions, defaults: FormattingOptions) {
+function mergeDefaults<V extends {}>(options: Partial<V>, defaults: V): V {
   const result = Object.create(defaults);
-  Object.keys(options).forEach((key) => {
+  Object.keys(options).forEach((untypedKey) => {
+    const key = untypedKey as keyof typeof options;
     let value = options[key];
     if (typeof value === 'object' && !Array.isArray(value)) {
-      value = mergeDefaults(value, defaults[key]);
+      value =
+          mergeDefaults(value as typeof defaults[typeof key], defaults[key]);
     }
     result[key] = value;
   });
   return result;
 }
 
-function methodPadding(lines: ParsedLine[], options: FormattingOptions) {
-  let size = options.methodPlaceholder.length;
+function methodPadding(
+    lines: Array<ParsedLine|null>, options: FormattingOptions) {
+  let size = (options.methodPlaceholder || '').length;
   for (let i = 0, line; line = lines[i]; i++) {
-    size =
-        Math.min(options.maxMethodPadding, Math.max(size, line.method.length));
+    size = Math.min(
+        options.maxMethodPadding || Infinity,
+        Math.max(size, line.method.length));
   }
   return size;
 }
diff --git a/packages/wct-mocha/src/stacktrace/normalization.ts b/packages/wct-mocha/src/stacktrace/normalization.ts
index 811041629..8673b6fc5 100644
--- a/packages/wct-mocha/src/stacktrace/normalization.ts
+++ b/packages/wct-mocha/src/stacktrace/normalization.ts
@@ -21,13 +21,14 @@ export type ErrorLikeThing = Error&{
 };
 
 export function normalize(
-    error: ErrorLikeThing, prettyOptions: FormattingOptions): ErrorLikeThing {
+    error: ErrorLikeThing,
+    prettyOptions: Partial<FormattingOptions>): ErrorLikeThing {
   if (error.parsedStack) {
     return error;
   }
   const message =
       error.message || error.description || error || '<unknown error>';
-  let parsedStack: ParsedLine[] = [];
+  let parsedStack: Array<ParsedLine|null> = [];
   try {
     parsedStack = parse(error.stack || error.toString());
   } catch (error) {
@@ -38,8 +39,8 @@ export function normalize(
     parsedStack.push({
       method: '',
       location: error.fileName,
-      line: error.lineNumber,
-      column: error.columnNumber,
+      line: error.lineNumber || -1,
+      column: error.columnNumber || -1,
     });
   }
 
diff --git a/packages/wct-mocha/src/stacktrace/parsing.ts b/packages/wct-mocha/src/stacktrace/parsing.ts
index 64321453e..e75bed43f 100644
--- a/packages/wct-mocha/src/stacktrace/parsing.ts
+++ b/packages/wct-mocha/src/stacktrace/parsing.ts
@@ -42,7 +42,7 @@ export function parse(stack: string) {
 // https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Error/Stack
 const GECKO_LINE = /^(?:([^@]*)@)?(.*?):(\d+)(?::(\d+))?$/;
 
-export function parseGeckoLine(line: string): ParsedLine {
+export function parseGeckoLine(line: string): ParsedLine|null {
   const match = line.match(GECKO_LINE);
   if (!match) {
     return null;
@@ -60,7 +60,7 @@ const V8_OUTER1 = /^\s*(eval )?at (.*) \((.*)\)$/;
 const V8_OUTER2 = /^\s*at()() (\S+)$/;
 const V8_INNER = /^\(?([^\(]+):(\d+):(\d+)\)?$/;
 
-export function parseV8Line(line: string): ParsedLine {
+export function parseV8Line(line: string): ParsedLine|null {
   const outer = line.match(V8_OUTER1) || line.match(V8_OUTER2);
   if (!outer) {
     return null;
@@ -83,7 +83,7 @@ export function parseV8Line(line: string): ParsedLine {
 
 const STACKY_LINE = /^\s*(.+) at (.+):(\d+):(\d+)$/;
 
-export function parseStackyLine(line: string): ParsedLine {
+export function parseStackyLine(line: string): ParsedLine|null {
   const match = line.match(STACKY_LINE);
   if (!match) {
     return null;
diff --git a/packages/wct-mocha/src/suites.ts b/packages/wct-mocha/src/suites.ts
index b61445c92..41a893119 100644
--- a/packages/wct-mocha/src/suites.ts
+++ b/packages/wct-mocha/src/suites.ts
@@ -47,7 +47,7 @@ export function loadSuites(files: string[]) {
 export function activeChildSuites(): string[] {
   let subsuites = htmlSuites;
   if (GREP) {
-    const cleanSubsuites = [];
+    const cleanSubsuites: string[] = [];
     for (let i = 0, subsuite; subsuite = subsuites[i]; i++) {
       if (GREP.indexOf(util.cleanLocation(subsuite)) !== -1) {
         cleanSubsuites.push(subsuite);
@@ -62,7 +62,7 @@ export function activeChildSuites(): string[] {
  * Loads all `.js` sources requested by the current suite.
  */
 export function loadJsSuites(
-    _reporter: MultiReporter, done: (error: Error) => void) {
+    _reporter: MultiReporter, done: (error: {}|undefined) => void) {
   util.debug('loadJsSuites', jsSuites);
 
   // We only support `.js` dependencies for now.
@@ -77,7 +77,7 @@ export function runSuites(
     done: (error?: {}) => void) {
   util.debug('runSuites');
 
-  const suiteRunners: Array<(next: () => void) => void> = [
+  const suiteRunners: Array<util.Runner> = [
     // Run the local tests (if any) first, not stopping on error;
     _runMocha.bind(null, reporter),
   ];
@@ -120,10 +120,9 @@ function _runMocha(reporter: MultiReporter, done: () => void, waited: boolean) {
   const mocha = window.mocha;
   const Mocha = window.Mocha;
 
-  mocha.reporter(
-      reporter.childReporter(window.location) as {} as Mocha.Reporter);
+  mocha.reporter(reporter.childReporter() as {} as Mocha.Reporter);
   mocha.suite.title = reporter.suiteTitle(window.location);
-  mocha.grep(GREP);
+  mocha.grep(GREP!);
 
   // We can't use `mocha.run` because it bashes over grep, invert, and friends.
   // See https://github.com/visionmedia/mocha/blob/master/support/tail.js#L137
@@ -139,7 +138,7 @@ function _runMocha(reporter: MultiReporter, done: () => void, waited: boolean) {
   //
   // TODO(nevir): Can we expand support to other browsers?
   if (navigator.userAgent.match(/chrome/i)) {
-    window.onerror = null;
+    window.onerror = null!;
     window.addEventListener('error', (event) => {
       if (!event.error) {
         return;
@@ -154,7 +153,7 @@ function _runMocha(reporter: MultiReporter, done: () => void, waited: boolean) {
       runner.uncaught(event.error);
     });
   } else {
-    window.onerror = null;
+    window.onerror = null!;
     window.addEventListener('error', (event) => {
       if (window.uncaughtErrorFilter && window.uncaughtErrorFilter(event)) {
         event.preventDefault();
diff --git a/packages/wct-mocha/src/util.ts b/packages/wct-mocha/src/util.ts
index f88173d6b..cc447a264 100644
--- a/packages/wct-mocha/src/util.ts
+++ b/packages/wct-mocha/src/util.ts
@@ -57,7 +57,7 @@ export function loadScript(path: string, done: (error?: {}) => void) {
     script.onload = done.bind(null, null);
     script.onerror = done.bind(null, 'Failed to load script ' + script.src);
   }
-  document.head.appendChild(script);
+  document.head!.appendChild(script);
 }
 
 /**
@@ -72,7 +72,7 @@ export function loadStyle(path: string, done?: () => void) {
     link.onload = done.bind(null, null);
     link.onerror = done.bind(null, 'Failed to load stylesheet ' + link.href);
   }
-  document.head.appendChild(link);
+  document.head!.appendChild(link);
 }
 
 /**
@@ -94,7 +94,7 @@ export function debug(...var_args: unknown[]) {
  * @return {{base: string, params: string}}
  */
 export function parseUrl(url: string) {
-  const parts = url.match(/^(.*?)(?:\?(.*))?$/);
+  const parts = url.match(/^(.*?)(?:\?(.*))?$/)!;
   return {
     base: parts[1],
     params: getParams(parts[2] || ''),
@@ -204,7 +204,7 @@ function getPathName(location: Location|string): string {
 }
 
 export function basePath(location: Location|string) {
-  return getPathName(location).match(/^.*\//)[0];
+  return getPathName(location).match(/^.*\//)![0];
 }
 
 export function relativeLocation(location: Location|string, basePath: string) {
@@ -236,9 +236,11 @@ export type Runner = (f: Function) => void;
  * @param {?function(*)} done Callback that should be triggered once all runners
  *     have completed, or encountered an error.
  */
-export function parallel(runners: Runner[], done: (error?: {}) => void): void;
 export function parallel(
-    runners: Runner[], limit: number, done: (error?: {}) => void): void;
+    runners: Runner[], done: (error?: {}|undefined) => void): void;
+export function parallel(
+    runners: Runner[], limit: number, done: (error?: {}|undefined) => void):
+    void;
 export function parallel(
     runners: Runner[],
     maybeLimit: number|((error?: {}) => void),
@@ -251,7 +253,7 @@ export function parallel(
     limit = maybeLimit;
   }
   if (!runners.length) {
-    return done();
+    return done && done();
   }
 
   let called = false;
@@ -268,7 +270,7 @@ export function parallel(
 
     if (error || numDone >= total) {
       called = true;
-      done(error);
+      done && done(error);
     } else {
       runOne();
     }
@@ -282,7 +284,7 @@ export function parallel(
       return;
     }
     numActive = numActive + 1;
-    runners.shift()(runnerDone);
+    runners.shift()!(runnerDone);
   }
   runners.forEach(runOne);
 }
diff --git a/tools.code-workspace b/tools.code-workspace
index 634b8cef6..330f08475 100644
--- a/tools.code-workspace
+++ b/tools.code-workspace
@@ -42,6 +42,9 @@
 		{
 			"path": "packages/wct-local"
 		},
+		{
+			"path": "packages/wct-mocha"
+		},
 		{
 			"path": "packages/web-component-tester"
 		},