Move extended asyncRequire to open source

Summary:
Changelog:

* **[Feature]**: Support custom `__loadBundleAsync` implementations in the default `asyncRequire` function. See the [lazy bundling RFC](https://github.com/react-native-community/discussions-and-proposals/blob/main/proposals/0605-lazy-bundling.md) for more details.

Reviewed By: huntie

Differential Revision: D43600050

fbshipit-source-id: f99b5c4f33bd39be522e072d5f4744122277355c

packages/metro-runtime/src/modules/asyncRequire.js

@@ -4,14 +4,75 @@
  * This source code is licensed under the MIT license found in the
  * LICENSE file in the root directory of this source tree.
  *
+ * @flow strict-local
  * @format
- * @flow strict
+ * @oncall react_native
  */
 
-'use strict';
+type Options = {isPrefetchOnly: boolean, ...};
+type MetroRequire = {
+  (number): mixed,
+  importAll: number => mixed,
+  ...
+};
+
+declare var require: MetroRequire;
+
+const DEFAULT_OPTIONS = {isPrefetchOnly: false};
+
+type DependencyMapPaths = ?$ReadOnly<{[moduleID: number | string]: mixed}>;
+
+declare var __METRO_GLOBAL_PREFIX__: string;
+
+async function asyncRequireImpl(
+  moduleID: number,
+  paths: DependencyMapPaths,
+  options: Options,
+): Promise<mixed> {
+  const loadBundle: (bundlePath: mixed) => Promise<void> =
+    global[`${__METRO_GLOBAL_PREFIX__}__loadBundleAsync`];
 
-// $FlowExpectedError Flow does not know about Metro's require extensions.
-const dynamicRequire = (require: {importAll: mixed => mixed});
-module.exports = function (moduleID: mixed): Promise<mixed> {
-  return Promise.resolve().then(() => dynamicRequire.importAll(moduleID));
+  if (loadBundle != null) {
+    const stringModuleID = String(moduleID);
+    const bundlePath = nullthrows(paths)[stringModuleID];
+    if (bundlePath != null) {
+      // NOTE: Errors will be swallowed by asyncRequire.prefetch
+      await loadBundle(bundlePath);
+    }
+  }
+
+  if (!options.isPrefetchOnly) {
+    return require.importAll(moduleID);
+  }
+
+  return undefined;
+}
+
+async function asyncRequire(
+  moduleID: number,
+  paths: DependencyMapPaths,
+  moduleName?: string,
+): Promise<mixed> {
+  return asyncRequireImpl(moduleID, paths, DEFAULT_OPTIONS);
+}
+
+asyncRequire.prefetch = function (
+  moduleID: number,
+  paths: DependencyMapPaths,
+  moduleName?: string,
+): void {
+  asyncRequireImpl(moduleID, paths, {isPrefetchOnly: true}).then(
+    () => {},
+    () => {},
+  );
 };
+
+module.exports = asyncRequire;
+
+// Inline definition to save on a dependency
+function nullthrows<T>(value: ?T): T {
+  if (value == null) {
+    throw new Error('Unexpected null or undefined');
+  }
+  return value;
+}