fix #626, add AMD module format and make it default

lib/src/js_ast/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.
+ * 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) {}
 }