diff --git a/packages/cli/src/tools/assign.js b/packages/cli/src/tools/assign.ts
similarity index 70%
rename from packages/cli/src/tools/assign.js
rename to packages/cli/src/tools/assign.ts
index 18f061712..621065bf7 100644
--- a/packages/cli/src/tools/assign.js
+++ b/packages/cli/src/tools/assign.ts
@@ -4,14 +4,19 @@
  * Similar to Object.assign(), but it doesn't execute getters. This allows us to have
  * lazy properties on an object and still be able to merge them together
  *
- * @flow
  */
 export default function assign(target: Object, ...sources: Object[]) {
   sources.forEach(source => {
-    let descriptors = Object.keys(source).reduce((acc, key) => {
-      acc[key] = Object.getOwnPropertyDescriptor(source, key);
-      return acc;
-    }, {});
+    let descriptors = Object.keys(source).reduce(
+      (acc, key) => {
+        const propertyDescriptor = Object.getOwnPropertyDescriptor(source, key);
+        if (propertyDescriptor !== undefined) {
+          acc[key] = propertyDescriptor;
+        }
+        return acc;
+      },
+      {} as PropertyDescriptorMap,
+    );
     // by default, Object.assign copies enumerable Symbols too
     Object.getOwnPropertySymbols(source).forEach(sym => {
       let descriptor = Object.getOwnPropertyDescriptor(source, sym);
diff --git a/packages/cli/src/tools/config/index.js b/packages/cli/src/tools/config/index.js
index eda09c404..1037c0a70 100644
--- a/packages/cli/src/tools/config/index.js
+++ b/packages/cli/src/tools/config/index.js
@@ -19,6 +19,7 @@ import type {
   UserConfigT,
   DependencyConfigT,
 } from 'types';
+// $FlowFixMe - converted to TS
 import assign from '../assign';
 // $FlowFixMe - converted to TS
 import merge from '../merge';
@@ -46,7 +47,7 @@ function getDependencyConfig(
               : platformConfig.dependencyConfig(
                   root,
                   /* $FlowFixMe - can't figure out which platform's dependency
-                   config to choose */
+                 config to choose */
                   config.dependency.platforms[platform],
                 );
           return dependency;
diff --git a/packages/cli/src/tools/isValidPackageName.js b/packages/cli/src/tools/isValidPackageName.js
deleted file mode 100644
index fcdf02334..000000000
--- a/packages/cli/src/tools/isValidPackageName.js
+++ /dev/null
@@ -1,14 +0,0 @@
-/**
- * Copyright (c) Facebook, Inc. and its affiliates.
- *
- * This source code is licensed under the MIT license found in the
- * LICENSE file in the root directory of this source tree.
- *
- * @format
- */
-
-function isValidPackageName(name) {
-  return name.match(/^[$A-Z_][0-9A-Z_$]*$/i);
-}
-
-export default isValidPackageName;