Check in a forked version of object-assign only for UMD builds (#18180)

* Check in a forked version of object-assign

This one uses ES modules so that we can inline it into UMD builds.

We could wait for object-assign to make an ESM export but we're going to
remove this dependency and assume global polyfills in the next version
anyway. However, we'd have to figure out how to keep the copyright header
and it'll get counted in terms of byte size (even if other tooling removes
it).

A lot of headache when we have our own implementation anyway. So I'll just
use that.

Ours is not resilient to checking certain browser bugs but those browsers
are mostly unused anyway. (Even FB breaks on them presumably.)

We also don't need to be resilient to Symbols since the way React uses it
we shouldn't need to copy symbols

* Don't transpile Object.assign to object-assign in object-assign

The polyfill needs to be able to feature detect Object.assign.

packages/shared/forks/object-assign.inline-umd.js

@@ -0,0 +1,31 @@
+/**
+ * Copyright 2004-present Facebook. All Rights Reserved.
+ */
+
+const hasOwnProperty = Object.prototype.hasOwnProperty;
+
+const _assign = function(to, from) {
+  for (let key in from) {
+    if (hasOwnProperty.call(from, key)) {
+      to[key] = from[key];
+    }
+  }
+};
+
+export default Object.assign ||
+  function(target, sources) {
+    if (target == null) {
+      throw new TypeError('Object.assign target cannot be null or undefined');
+    }
+
+    const to = Object(target);
+
+    for (let nextIndex = 1; nextIndex < arguments.length; nextIndex++) {
+      const nextSource = arguments[nextIndex];
+      if (nextSource != null) {
+        _assign(to, Object(nextSource));
+      }
+    }
+
+    return to;
+  };

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

@@ -28,6 +28,10 @@ module.exports = function autoImporter(babel) {
 
     visitor: {
       CallExpression: function(path, file) {
+        if (file.filename.indexOf('object-assign') !== -1) {
+          // Don't replace Object.assign if we're transforming object-assign
+          return;
+        }
         if (path.get('callee').matchesPattern('Object.assign')) {
           // generate identifier and require if it hasn't been already
           const id = getAssignIdent(path, file, this);
@@ -36,6 +40,10 @@ module.exports = function autoImporter(babel) {
       },
 
       MemberExpression: function(path, file) {
+        if (file.filename.indexOf('object-assign') !== -1) {
+          // Don't replace Object.assign if we're transforming object-assign
+          return;
+        }
         if (path.matchesPattern('Object.assign')) {
           const id = getAssignIdent(path, file, this);
           path.replaceWith(id);

scripts/rollup/forks.js

@@ -22,8 +22,8 @@ const {RENDERER, RECONCILER} = moduleTypes;
 // If you need to replace a file with another file for a specific environment,
 // add it to this list with the logic for choosing the right replacement.
 const forks = Object.freeze({
-  // Optimization: for UMDs, use object-assign polyfill that is already a part
-  // of the React package instead of bundling it again.
+  // Optimization: for UMDs, use a version that we can inline into the React bundle.
+  // Use that from all other bundles.
   'object-assign': (bundleType, entry, dependencies) => {
     if (
       bundleType !== UMD_DEV &&
@@ -34,12 +34,16 @@ const forks = Object.freeze({
       // happens. Other bundles just require('object-assign') anyway.
       return null;
     }
+    if (entry === 'react') {
+      // Use the forked version that uses ES modules instead of CommonJS.
+      return 'shared/forks/object-assign.inline-umd.js';
+    }
     if (dependencies.indexOf('react') === -1) {
       // We can only apply the optimizations to bundle that depend on React
       // because we read assign() from an object exposed on React internals.
       return null;
     }
-    // We can use the fork!
+    // We can use the fork that reads the secret export!
     return 'shared/forks/object-assign.umd.js';
   },