diff --git a/CHANGELOG.md b/CHANGELOG.md
index a3427cf93..0f9a687d4 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,12 @@
+## 0.12.25
+
+* Add a `override_platforms` configuration field which allows test platforms'
+  settings (such as browsers' executables) to be overridden by the user.
+
+* Add a `define_platforms` configuration field which makes it possible to define
+  new platforms that use the same logic as existing ones but have different
+  settings.
+
 ## 0.12.24+8
 
 * `spawnHybridUri()` now interprets relative URIs correctly in browser tests.
diff --git a/doc/configuration.md b/doc/configuration.md
index d3d78d505..c95fa004e 100644
--- a/doc/configuration.md
+++ b/doc/configuration.md
@@ -52,7 +52,8 @@ tags:
 * [Configuring Platforms](#configuring-platforms)
   * [`on_os`](#on_os)
   * [`on_platform`](#on_platform)
-  * [`define_platform`](#define_platform)
+  * [`override_platforms`](#override_platforms)
+  * [`define_platforms`](#define_platforms)
   * [Browser Settings](#browser-settings)
     * [`executable`](#executable)
 * [Configuration Presets](#configuration-presets)
@@ -570,6 +571,26 @@ when running on a particular operating system, use [`on_os`](#on_os) instead.
 
 This field counts as [test configuration](#test-configuration).
 
+### `override_platforms`
+
+This field allows you to customize the settings for built-in test platforms. It
+takes a map from platform identifiers to settings for those platforms. For example:
+
+```yaml
+override_platforms:
+  chrome:
+    # The settings to override for this platform.
+    settings:
+      executable: chromium
+```
+
+This tells the test runner to use the `chromium` executable for Chrome tests. It
+calls that executable with the same logic and flags it normally uses for Chrome.
+
+Each platform can define exactly which settings it supports. All browsers
+support [the same settings](#browser-settings), but the VM doesn't support any
+settings and so can't be overridden.
+
 ### `define_platforms`
 
 You can define new platforms in terms of old ones using the `define_platforms`
diff --git a/lib/src/runner/browser/platform.dart b/lib/src/runner/browser/platform.dart
index 776f1be91..02c1984de 100644
--- a/lib/src/runner/browser/platform.dart
+++ b/lib/src/runner/browser/platform.dart
@@ -202,6 +202,9 @@ class BrowserPlatform extends PlatformPlugin
   ExecutableSettings parsePlatformSettings(YamlMap settings) =>
       new ExecutableSettings.parse(settings);
 
+  ExecutableSettings mergePlatformSettings(ExecutableSettings settings1, ExecutableSettings settings2) =>
+      settings1.merge(settings2);
+
   void customizePlatform(TestPlatform platform, ExecutableSettings settings) {
     var oldSettings =
         _browserSettings[platform] ?? _browserSettings[platform.root];
diff --git a/lib/src/runner/configuration.dart b/lib/src/runner/configuration.dart
index acd8c6567..bba0fa78f 100644
--- a/lib/src/runner/configuration.dart
+++ b/lib/src/runner/configuration.dart
@@ -8,6 +8,7 @@ import 'package:boolean_selector/boolean_selector.dart';
 import 'package:collection/collection.dart';
 import 'package:glob/glob.dart';
 import 'package:path/path.dart' as p;
+import 'package:source_span/source_span.dart';
 
 import '../backend/platform_selector.dart';
 import '../backend/test_platform.dart';
@@ -18,6 +19,7 @@ import 'configuration/args.dart' as args;
 import 'configuration/custom_platform.dart';
 import 'configuration/load.dart';
 import 'configuration/platform_selection.dart';
+import 'configuration/platform_settings.dart';
 import 'configuration/reporters.dart';
 import 'configuration/suite.dart';
 import 'configuration/values.dart';
@@ -172,6 +174,9 @@ class Configuration {
 
   Set<String> _knownPresets;
 
+  /// Built-in platforms whose settings are overridden by the user.
+  final Map<String, PlatformSettings> overridePlatforms;
+
   /// Platforms defined by the user in terms of existing platforms.
   final Map<String, CustomPlatform> definePlatforms;
 
@@ -218,6 +223,7 @@ class Configuration {
       Glob filename,
       Iterable<String> chosenPresets,
       Map<String, Configuration> presets,
+      Map<String, PlatformSettings> overridePlatforms,
       Map<String, CustomPlatform> definePlatforms,
       bool noRetry,
 
@@ -261,6 +267,7 @@ class Configuration {
         filename: filename,
         chosenPresets: chosenPresetSet,
         presets: _withChosenPresets(presets, chosenPresetSet),
+        overridePlatforms: overridePlatforms,
         definePlatforms: definePlatforms,
         noRetry: noRetry,
         suiteDefaults: new SuiteConfiguration(
@@ -316,6 +323,7 @@ class Configuration {
       Glob filename,
       Iterable<String> chosenPresets,
       Map<String, Configuration> presets,
+      Map<String, PlatformSettings> overridePlatforms,
       Map<String, CustomPlatform> definePlatforms,
       bool noRetry,
       SuiteConfiguration suiteDefaults})
@@ -337,6 +345,7 @@ class Configuration {
         chosenPresets =
             new UnmodifiableSetView(chosenPresets?.toSet() ?? new Set()),
         presets = _map(presets),
+        overridePlatforms = _map(overridePlatforms),
         definePlatforms = _map(definePlatforms),
         _noRetry = noRetry,
         suiteDefaults = pauseAfterLoad == true
@@ -399,6 +408,13 @@ class Configuration {
     // We don't need to verify [customPlatforms] here because those platforms
     // already need to be verified and resolved to create [allPlatforms].
 
+    for (var settings in overridePlatforms.values) {
+      if (!allPlatforms.any((platform) => platform.identifier == settings.identifier)) {
+        throw new SourceSpanFormatException(
+            'Unknown platform "${settings.identifier}".', settings.identifierSpan);
+      }
+    }
+
     suiteDefaults.validatePlatforms(allPlatforms);
     for (var config in presets.values) {
       config.validatePlatforms(allPlatforms);
@@ -448,6 +464,12 @@ class Configuration {
         filename: other._filename ?? _filename,
         chosenPresets: chosenPresets.union(other.chosenPresets),
         presets: _mergeConfigMaps(presets, other.presets),
+        overridePlatforms: mergeUnmodifiableMaps(
+            overridePlatforms, other.overridePlatforms,
+            value: (settings1, settings2) => new PlatformSettings(
+                settings1.identifier,
+                settings1.identifierSpan,
+                settings1.settings.toList()..addAll(settings2.settings))),
         definePlatforms:
             mergeUnmodifiableMaps(definePlatforms, other.definePlatforms),
         noRetry: other._noRetry ?? _noRetry,
@@ -482,6 +504,7 @@ class Configuration {
       Glob filename,
       Iterable<String> chosenPresets,
       Map<String, Configuration> presets,
+        Map<String, PlatformSettings> overridePlatforms,
       Map<String, CustomPlatform> definePlatforms,
       bool noRetry,
 
@@ -523,6 +546,7 @@ class Configuration {
         filename: filename ?? _filename,
         chosenPresets: chosenPresets ?? this.chosenPresets,
         presets: presets ?? this.presets,
+        overridePlatforms: overridePlatforms ?? this.overridePlatforms,
         definePlatforms: definePlatforms ?? this.definePlatforms,
         noRetry: noRetry ?? _noRetry,
         suiteDefaults: suiteDefaults.change(
diff --git a/lib/src/runner/configuration/load.dart b/lib/src/runner/configuration/load.dart
index 705e16d0e..68695e679 100644
--- a/lib/src/runner/configuration/load.dart
+++ b/lib/src/runner/configuration/load.dart
@@ -20,6 +20,7 @@ import '../configuration.dart';
 import '../configuration/suite.dart';
 import 'custom_platform.dart';
 import 'platform_selection.dart';
+import 'platform_settings.dart';
 import 'reporters.dart';
 
 /// A regular expression matching a Dart identifier.
@@ -193,6 +194,7 @@ class _ConfigurationLoader {
       _disallow("plain_names");
       _disallow("platforms");
       _disallow("add_presets");
+      _disallow("override_platforms");
       return Configuration.empty;
     }
 
@@ -215,13 +217,42 @@ class _ConfigurationLoader {
     var chosenPresets = _getList("add_presets",
         (presetNode) => _parseIdentifierLike(presetNode, "Preset name"));
 
+    var overridePlatforms = _loadOverridePlatforms();
+
     return new Configuration(
         pauseAfterLoad: pauseAfterLoad,
         runSkipped: runSkipped,
         reporter: reporter,
         concurrency: concurrency,
         platforms: platforms,
-        chosenPresets: chosenPresets);
+        chosenPresets: chosenPresets,
+        overridePlatforms: overridePlatforms);
+  }
+
+  /// Loads the `override_platforms` field.
+  Map<String, PlatformSettings> _loadOverridePlatforms() {
+    var platformsNode =
+        _getNode("override_platforms", "map", (value) => value is Map) as YamlMap;
+    if (platformsNode == null) return const {};
+
+    var platforms = <String, PlatformSettings>{};
+    platformsNode.nodes.forEach((identifierNode, valueNode) {
+      var identifier =
+          _parseIdentifierLike(identifierNode, "Platform identifier");
+
+      _validate(valueNode, "Platform definition must be a map.",
+          (value) => value is Map);
+      var map = valueNode as YamlMap;
+
+      var settings = _expect(map, "settings");
+      _validate(settings, "Must be a map.", (value) => value is Map);
+
+      platforms[identifier] = new PlatformSettings(
+          identifier,
+          identifierNode.span,
+          [settings as YamlMap]);
+    });
+    return platforms;
   }
 
   /// Loads runner configuration that's not allowed in the global configuration
@@ -316,6 +347,7 @@ class _ConfigurationLoader {
     });
   }
 
+  /// Loads the `define_platforms` field.
   Map<String, CustomPlatform> _loadDefinePlatforms() {
     var platformsNode =
         _getNode("define_platforms", "map", (value) => value is Map) as YamlMap;
diff --git a/lib/src/runner/configuration/platform_settings.dart b/lib/src/runner/configuration/platform_settings.dart
new file mode 100644
index 000000000..203901564
--- /dev/null
+++ b/lib/src/runner/configuration/platform_settings.dart
@@ -0,0 +1,26 @@
+// Copyright (c) 2017, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import 'package:source_span/source_span.dart';
+import 'package:yaml/yaml.dart';
+
+import '../plugin/customizable_platform.dart';
+
+/// User-defined settings for a built-in test platform.
+class PlatformSettings {
+  /// The identifier used to look up the platform being overridden.
+  final String identifier;
+
+  /// The location that [identifier] was defined in the configuration file.
+  final SourceSpan identifierSpan;
+
+  /// The user's settings for this platform.
+  ///
+  /// This is a list of settings, from most global to most specific, that will
+  /// eventually be merged using [CustomizablePlatform.mergePlatformSettings].
+  final List<YamlMap> settings;
+
+  PlatformSettings(this.identifier, this.identifierSpan, List<YamlMap> settings)
+      : settings = new List.unmodifiable(settings);
+}
diff --git a/lib/src/runner/loader.dart b/lib/src/runner/loader.dart
index 4c71321b3..16c146964 100644
--- a/lib/src/runner/loader.dart
+++ b/lib/src/runner/loader.dart
@@ -53,8 +53,9 @@ class Loader {
   /// their string identifiers.
   final _platformsByIdentifier = <String, TestPlatform>{};
 
-  /// The user-provided settings for platforms.
-  final _platformSettings = <TestPlatform, YamlMap>{};
+  /// The user-provided settings for platforms, as a list of settings that will
+  /// be merged together using [CustomizablePlatform.mergePlatformSettings].
+  final _platformSettings = <TestPlatform, List<YamlMap>>{};
 
   /// The user-provided settings for platforms.
   final _parsedPlatformSettings = <TestPlatform, Object>{};
@@ -104,6 +105,8 @@ class Loader {
     _registerCustomPlatforms();
 
     _config.validatePlatforms(allPlatforms);
+
+    _registerPlatformOverrides();
   }
 
   /// Registers a [PlatformPlugin] for [platforms].
@@ -140,7 +143,19 @@ class Loader {
       _platformCallbacks[platform] = _platformCallbacks[parent];
       _platformsByIdentifier[platform.identifier] = platform;
 
-      _platformSettings[platform] = customPlatform.settings;
+      _platformSettings[platform] = [customPlatform.settings];
+    }
+  }
+
+  /// Registers users' platform settings from [Configuration.overridePlatforms].
+  void _registerPlatformOverrides() {
+    for (var settings in _config.overridePlatforms.values) {
+      var platform = _platformsByIdentifier[settings.identifier];
+
+      // This is officially validated in [Configuration.validatePlatforms].
+      assert(platform != null);
+
+      _platformSettings.putIfAbsent(platform, () => []).addAll(settings.settings);
     }
   }
 
@@ -258,7 +273,7 @@ class Loader {
     if (settings == null) return;
 
     if (plugin is CustomizablePlatform) {
-      parsed = plugin.parsePlatformSettings(settings);
+      parsed = settings.map(plugin.parsePlatformSettings).reduce(plugin.mergePlatformSettings);
       plugin.customizePlatform(platform, parsed);
       _parsedPlatformSettings[platform] = parsed;
     } else {
@@ -268,9 +283,8 @@ class Loader {
         identifier = platform.parent.identifier;
         span = _config.definePlatforms[platform.identifier].parentSpan;
       } else {
-        // TODO(nweiz): Get information from [_config.overridePlatforms] when
-        // that exists.
-        throw new UnimplementedError();
+        identifier = platform.identifier;
+        span = _config.overridePlatforms[platform.identifier].identifierSpan;
       }
 
       throw new SourceSpanFormatException(
diff --git a/lib/src/runner/plugin/customizable_platform.dart b/lib/src/runner/plugin/customizable_platform.dart
index d8a2ce582..e1ebd57e2 100644
--- a/lib/src/runner/plugin/customizable_platform.dart
+++ b/lib/src/runner/plugin/customizable_platform.dart
@@ -34,6 +34,15 @@ abstract class CustomizablePlatform<T> extends PlatformPlugin {
   /// possible.
   T parsePlatformSettings(YamlMap settings);
 
+  /// Merges [settings1] with [settings2] and returns a new settings object that
+  /// includes the configuration of both.
+  ///
+  /// When the settings conflict, [settings2] should take priority.
+  ///
+  /// This is used to merge global settings with local settings, or a custom
+  /// platform's settings with its parent's.
+  T mergePlatformSettings(T settings1, T settings2);
+
   /// Defines user-provided [settings] for [platform].
   ///
   /// The [platform] is a platform this plugin was declared to accept when
diff --git a/lib/src/utils.dart b/lib/src/utils.dart
index 687cd1a3e..2e06ef1c1 100644
--- a/lib/src/utils.dart
+++ b/lib/src/utils.dart
@@ -199,10 +199,11 @@ List flatten(Iterable nested) {
 /// creating a new map unnecessarily.
 ///
 /// The return value *may or may not* be unmodifiable.
-Map<K, V> mergeUnmodifiableMaps<K, V>(Map<K, V> map1, Map<K, V> map2) {
+Map<K, V> mergeUnmodifiableMaps<K, V>(Map<K, V> map1, Map<K, V> map2,
+    {V value(V value1, V value2)}) {
   if (map1.isEmpty) return map2;
   if (map2.isEmpty) return map1;
-  return mergeMaps(map1, map2);
+  return mergeMaps(map1, map2, value: value);
 }
 
 /// Truncates [text] to fit within [maxLength].
diff --git a/pubspec.yaml b/pubspec.yaml
index 86f72f713..764d2e816 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -1,5 +1,5 @@
 name: test
-version: 0.12.24+8
+version: 0.12.25-dev
 author: Dart Team <misc@dartlang.org>
 description: A library for writing dart unit tests.
 homepage: https://github.com/dart-lang/test
diff --git a/test/runner/configuration/custom_platform_test.dart b/test/runner/configuration/custom_platform_test.dart
index 45bf15c27..18efc25d3 100644
--- a/test/runner/configuration/custom_platform_test.dart
+++ b/test/runner/configuration/custom_platform_test.dart
@@ -27,17 +27,328 @@ void main() {
       """).create();
   });
 
-  group("define_platforms", () {
-    group("can define a new browser", () {
-      group("without any changes", () {
-        setUp(() async {
-          await d.file("dart_test.yaml", """
+  group("override_platforms", () {
+    group("can override a browser", () {
+      test("without any changes", () async {
+        await d.file("dart_test.yaml", """
+          override_platforms:
+            chrome:
+              settings: {}
+        """).create();
+
+        var test = await runTest(["-p", "chrome", "test.dart"]);
+        expect(test.stdout, emitsThrough(contains("All tests passed!")));
+        await test.shouldExit(0);
+      }, tags: "chrome");
+
+      test("that's user-defined", () async {
+        await d.file("dart_test.yaml", """
           define_platforms:
             chromium:
               name: Chromium
               extends: chrome
               settings: {}
+
+          override_platforms:
+            chromium:
+              settings: {}
+        """).create();
+
+        var test = await runTest(["-p", "chromium", "test.dart"]);
+        expect(test.stdout, emitsThrough(contains("All tests passed!")));
+        await test.shouldExit(0);
+      }, tags: "chrome");
+
+      test("with a basename-only executable", () async {
+        await d.file("dart_test.yaml", """
+          override_platforms:
+            dartium:
+              settings:
+                executable:
+                  linux: dartium
+                  mac_os: dartium
+                  windows: dartium.exe
+        """).create();
+
+        var test = await runTest(["-p", "dartium", "test.dart"]);
+        expect(test.stdout, emitsThrough(contains("All tests passed!")));
+        await test.shouldExit(0);
+      }, tags: "dartium");
+
+      test("with an absolute-path executable", () async {
+        String path;
+        if (Platform.isLinux) {
+          var process = await TestProcess.start("which", ["google-chrome"]);
+          path = await process.stdout.next;
+          await process.shouldExit(0);
+        } else {
+          path = defaultSettings[TestPlatform.chrome].executable;
+        }
+
+        await d.file("dart_test.yaml", """
+          override_platforms:
+            chrome:
+              settings:
+                executable: $path
+        """).create();
+
+        var test = await runTest(["-p", "chrome", "test.dart"]);
+        expect(test.stdout, emitsThrough(contains("All tests passed!")));
+        await test.shouldExit(0);
+      }, tags: "chrome");
+    });
+
+    group("errors", () {
+      test("rejects a non-map value", () async {
+        await d.file("dart_test.yaml", "override_platforms: 12").create();
+
+        var test = await runTest([]);
+        expect(test.stderr,
+            containsInOrder(["override_platforms must be a map.", "^^"]));
+        await test.shouldExit(exit_codes.data);
+      });
+
+      test("rejects a non-string key", () async {
+        await d.file("dart_test.yaml", "override_platforms: {12: null}").create();
+
+        var test = await runTest([]);
+        expect(test.stderr,
+            containsInOrder(["Platform identifier must be a string.", "^^"]));
+        await test.shouldExit(exit_codes.data);
+      });
+
+      test("rejects a non-identifier-like key", () async {
+        await d
+            .file("dart_test.yaml", "override_platforms: {foo bar: null}")
+            .create();
+
+        var test = await runTest([]);
+        expect(
+            test.stderr,
+            containsInOrder([
+              "Platform identifier must be an (optionally hyphenated) Dart "
+                  "identifier.",
+              "^^^^^^^"
+            ]));
+        await test.shouldExit(exit_codes.data);
+      });
+
+      test("rejects a non-map definition", () async {
+        await d.file("dart_test.yaml", """
+          override_platforms:
+            chrome: 12
+        """).create();
+
+        var test = await runTest([]);
+        expect(test.stderr,
+            containsInOrder(["Platform definition must be a map.", "^^"]));
+        await test.shouldExit(exit_codes.data);
+      });
+
+      test("requires a settings key", () async {
+        await d.file("dart_test.yaml", """
+          override_platforms:
+            chrome: {}
+        """).create();
+
+        var test = await runTest([]);
+        expect(
+            test.stderr,
+            containsInOrder(
+                ['Missing required field "settings".', "^^"]));
+        await test.shouldExit(exit_codes.data);
+      });
+
+      test("settings must be a map", () async {
+        await d.file("dart_test.yaml", """
+          override_platforms:
+            chrome:
+              settings: null
+        """).create();
+
+        var test = await runTest([]);
+        expect(test.stderr, containsInOrder(['Must be a map.', "^^^^"]));
+        await test.shouldExit(exit_codes.data);
+      });
+
+      test("the overridden platform must exist", () async {
+        await d.file("dart_test.yaml", """
+          override_platforms:
+            chromium:
+              settings: {}
+        """).create();
+
+        var test = await runTest(["test.dart"]);
+        expect(
+            test.stderr,
+            containsInOrder([
+              'Unknown platform "chromium".',
+              "^^^^^^"
+            ]));
+        await test.shouldExit(exit_codes.data);
+      });
+
+      test("uncustomizable platforms can't be overridden",
+          () async {
+        await d.file("dart_test.yaml", """
+          override_platforms:
+            vm:
+              settings: {}
         """).create();
+
+        var test = await runTest(["-p", "vm", "test.dart"]);
+        expect(test.stdout,
+            containsInOrder(['The "vm" platform can\'t be customized.', "^^"]));
+        await test.shouldExit(1);
+      });
+
+      group("when overriding browsers", () {
+        test("executable must be a string or map", () async {
+          await d.file("dart_test.yaml", """
+            override_platforms:
+              chrome:
+                settings:
+                  executable: 12
+          """).create();
+
+          var test = await runTest(["-p", "chrome", "test.dart"]);
+          expect(test.stdout,
+              containsInOrder(['Must be a map or a string.', "^^"]));
+          await test.shouldExit(1);
+        });
+
+        test("executable string may not be relative on POSIX", () async {
+          await d.file("dart_test.yaml", """
+            override_platforms:
+              chrome:
+                settings:
+                  executable: foo/bar
+          """).create();
+
+          var test = await runTest(["-p", "chrome", "test.dart"]);
+          expect(
+              test.stdout,
+              containsInOrder([
+                'Linux and Mac OS executables may not be relative paths.',
+                "^^^^^^^"
+              ]));
+          await test.shouldExit(1);
+        },
+            // We allow relative executables for Windows so that Windows users
+            // can set a global executable without having to explicitly write
+            // `windows:`.
+            testOn: "!windows");
+
+        test("Linux executable must be a string", () async {
+          await d.file("dart_test.yaml", """
+            override_platforms:
+              chrome:
+                settings:
+                  executable:
+                    linux: 12
+          """).create();
+
+          var test = await runTest(["-p", "chrome", "test.dart"]);
+          expect(test.stdout, containsInOrder(['Must be a string.', "^^"]));
+          await test.shouldExit(1);
+        });
+
+        test("Linux executable may not be relative", () async {
+          await d.file("dart_test.yaml", """
+            override_platforms:
+              chrome:
+                settings:
+                  executable:
+                    linux: foo/bar
+          """).create();
+
+          var test = await runTest(["-p", "chrome", "test.dart"]);
+          expect(
+              test.stdout,
+              containsInOrder([
+                'Linux and Mac OS executables may not be relative paths.',
+                "^^^^^^^"
+              ]));
+          await test.shouldExit(1);
+        });
+
+        test("Mac OS executable must be a string", () async {
+          await d.file("dart_test.yaml", """
+            override_platforms:
+              chrome:
+                settings:
+                  executable:
+                    mac_os: 12
+          """).create();
+
+          var test = await runTest(["-p", "chrome", "test.dart"]);
+          expect(test.stdout, containsInOrder(['Must be a string.', "^^"]));
+          await test.shouldExit(1);
+        });
+
+        test("Mac OS executable may not be relative", () async {
+          await d.file("dart_test.yaml", """
+            override_platforms:
+              chrome:
+                settings:
+                  executable:
+                    mac_os: foo/bar
+          """).create();
+
+          var test = await runTest(["-p", "chrome", "test.dart"]);
+          expect(
+              test.stdout,
+              containsInOrder([
+                'Linux and Mac OS executables may not be relative paths.',
+                "^^^^^^^"
+              ]));
+          await test.shouldExit(1);
+        });
+
+        test("Windows executable must be a string", () async {
+          await d.file("dart_test.yaml", """
+            override_platforms:
+              chrome:
+                settings:
+                  executable:
+                    windows: 12
+          """).create();
+
+          var test = await runTest(["-p", "chrome", "test.dart"]);
+          expect(test.stdout, containsInOrder(['Must be a string.', "^^"]));
+          await test.shouldExit(1);
+        });
+
+        test("executable must exist", () async {
+          await d.file("dart_test.yaml", """
+            override_platforms:
+              chrome:
+                settings:
+                  executable: _does_not_exist
+          """).create();
+
+          var test = await runTest(["-p", "chrome", "test.dart"]);
+          expect(
+              test.stdout,
+              emitsThrough(
+                  contains("Failed to run Chrome: $noSuchFileMessage")));
+          await test.shouldExit(1);
+        });
+      });
+    });
+  });
+
+  group("define_platforms", () {
+    group("can define a new browser", () {
+      group("without any changes", () {
+        setUp(() async {
+          await d.file("dart_test.yaml", """
+            define_platforms:
+              chromium:
+                name: Chromium
+                extends: chrome
+                settings: {}
+          """).create();
         });
 
         test("that can be used to run tests", () async {
@@ -48,12 +359,12 @@ void main() {
 
         test("that can be used in platform selectors", () async {
           await d.file("test.dart", """
-        import 'package:test/test.dart';
+            import 'package:test/test.dart';
 
-        void main() {
-          test("success", () {}, testOn: "chromium");
-        }
-      """).create();
+            void main() {
+              test("success", () {}, testOn: "chromium");
+            }
+          """).create();
 
           var test = await runTest(["-p", "chromium", "test.dart"]);
           expect(test.stdout, emitsThrough(contains("All tests passed!")));
@@ -66,12 +377,12 @@ void main() {
 
         test("that counts as its parent", () async {
           await d.file("test.dart", """
-        import 'package:test/test.dart';
+            import 'package:test/test.dart';
 
-        void main() {
-          test("success", () {}, testOn: "chrome");
-        }
-      """).create();
+            void main() {
+              test("success", () {}, testOn: "chrome");
+            }
+          """).create();
 
           var test = await runTest(["-p", "chromium", "test.dart"]);
           expect(test.stdout, emitsThrough(contains("All tests passed!")));