Implement modular compilation

lib/src/js/module_transform.dart

-// Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
-// for details. All rights reserved. Use of this source code is governed by a
-// BSD-style license that can be found in the LICENSE file.
-
-part of js_ast;
-
-/**
- * Transforms EcmaScript 6 modules to an ES 5 file using a module pattern.
- *
- * There are various module patterns in JavaScript, see
- * <http://babeljs.io/docs/usage/modules/> for some examples.
- *
- * At the moment, we only support our "custom Dart" conversion, roughly similar
- * to Asynchronous Module Definition (AMD), see also
- * <http://requirejs.org/docs/whyamd.html>. Like AMD, module files can
- * be loaded directly in the browser with no further transformation (e.g.
- * browserify, webpack).
- */
-// TODO(jmesserly): deprecate the "custom dart" form in favor of AMD.
-class CustomDartModuleTransform extends BaseVisitor {
-  // TODO(jmesserly): implement these. Module should transform to Program.
-  visitImportDeclaration(ImportDeclaration node) {}
-  visitExportDeclaration(ExportDeclaration node) {}
-  visitModule(Module node) {}
-}