Transform Object.assign to now use shared/assign

We need this to use the shared instance when Object.spread is used.

scripts/babel/transform-object-assign.js

@@ -0,0 +1,54 @@
+/**
+ * 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.
+ */
+
+'use strict';
+
+const helperModuleImports = require('@babel/helper-module-imports');
+
+module.exports = function autoImporter(babel) {
+  function getAssignIdent(path, file, state) {
+    if (state.id) {
+      return state.id;
+    }
+    state.id = helperModuleImports.addDefault(path, 'shared/assign', {
+      nameHint: 'assign',
+    });
+    return state.id;
+  }
+
+  return {
+    pre: function() {
+      // map from module to generated identifier
+      this.id = null;
+    },
+
+    visitor: {
+      CallExpression: function(path, file) {
+        if (file.filename.indexOf('shared/assign') !== -1) {
+          // Don't replace Object.assign if we're transforming shared/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);
+          path.node.callee = id;
+        }
+      },
+
+      MemberExpression: function(path, file) {
+        if (file.filename.indexOf('shared/assign') !== -1) {
+          // Don't replace Object.assign if we're transforming shared/assign
+          return;
+        }
+        if (path.matchesPattern('Object.assign')) {
+          const id = getAssignIdent(path, file, this);
+          path.replaceWith(id);
+        }
+      },
+    },
+  };
+};

scripts/rollup/build.js

@@ -126,6 +126,8 @@ const babelPlugins = [
   '@babel/plugin-transform-parameters',
   // TODO: Remove array destructuring from the source. Requires runtime.
   ['@babel/plugin-transform-destructuring', {loose: true, useBuiltIns: true}],
+  // Transform Object spread to shared/assign
+  require('../babel/transform-object-assign'),
 ];
 
 const babelToES5Plugins = [