Make metro-bundler use a no-op transformer by default

Summary: When starting to use `metro-bundler` as a standalone server, I found quite complicated the fact that it needs a transformer by default. Moreover, the transformer is not passed as a reference to a JS object, but as a path to a required file. I removed the need to be required, and defaulted to a dummy, basic transform, that does nothing by default. This avoids having to create an extra file and linking to it (potentially needing to involve `path` and other extra modules).

Reviewed By: cpojer

Differential Revision: D5622906

fbshipit-source-id: 0c2b1bec86fa542b3c05de42c89d4b5bb4384b34

packages/metro-bundler/src/Server/index.js

@@ -91,7 +91,7 @@ export type Options = {|
   silent?: boolean,
   +sourceExts: ?Array<string>,
   +transformCache: TransformCache,
-  +transformModulePath: string,
+  transformModulePath?: string,
   watch?: boolean,
   workerPath: ?string,
 |};
@@ -214,7 +214,8 @@ class Server {
       silent: options.silent || false,
       sourceExts: options.sourceExts || defaults.sourceExts,
       transformCache: options.transformCache,
-      transformModulePath: options.transformModulePath,
+      transformModulePath:
+        options.transformModulePath || defaults.transformModulePath,
       watch: options.watch || false,
       workerPath: options.workerPath,
     };

packages/metro-bundler/src/defaultTransform.js

@@ -0,0 +1,13 @@
+/**
+ * Copyright (c) 2015-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.
+ *
+ * @flow
+ */
+'use strict';
+
+module.exports.transform = (file: {src: string}) => ({code: file.src});

packages/metro-bundler/src/defaults.js

@@ -29,6 +29,8 @@ exports.providesModuleNodeModules = [
   'react-native-windows',
 ];
 
+exports.transformModulePath = require.resolve('./defaultTransform.js');
+
 exports.runBeforeMainModule = [
   // Ensures essential globals are available and are patched correctly.
   'InitializeCore',