Use Object.assign directly and inject object-assign at compile

scripts/babel/transform-object-assign-require.js

@@ -0,0 +1,40 @@
+/**
+ * Copyright 2013-present, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ */
+
+'use strict';
+
+module.exports = function autoImporter(babel) {
+  const t = babel.types;
+
+  return {
+    pre: function() {
+      // map from module to generated identifier
+      this.id = null;
+    },
+
+    visitor: {
+      CallExpression: function(path, file) {
+        if (path.get('callee').matchesPattern('Object.assign')) {
+          // generate identifier and require if it hasn't been already
+          if (!this.id) {
+            this.id = path.scope.generateUidIdentifier('assign');
+            path.scope.getProgramParent().push({
+              id: this.id,
+              init: t.callExpression(
+                t.identifier('require'),
+                [t.stringLiteral('object-assign')]
+              ),
+            });
+          }
+          path.node.callee = this.id;
+        }
+      },
+    },
+  };
+};

scripts/jest/preprocessor.js

@@ -27,7 +27,15 @@ var pathToBabelrc = path.join(__dirname, '..', '..', '.babelrc');
 // TODO: make sure this stays in sync with gulpfile
 var babelOptions = {
   plugins: [
-    [babelPluginModules, { map: moduleMap }],
+    [babelPluginModules, {
+      map: Object.assign(
+        {},
+        moduleMap,
+        {
+          'object-assign': 'object-assign',
+        }
+      ),
+    }],
   ],
   retainLines: true,
 };