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

lib/src/js_ast/nodes.dart

 // Copyright (c) 2012, 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;
 
 abstract class NodeVisitor<T> implements TypeRefVisitor<T> {
   T visitProgram(Program node);
 
   T visitBlock(Block node);
@@ skipping 286 matching lines @@
   String toString() {
     var context = new SimpleJavaScriptPrintingContext();
     var opts = new JavaScriptPrintingOptions(allowKeywordsInProperties: true);
     context.buffer.write('js_ast `');
     accept(new Printer(opts, context));
     context.buffer.write('`');
     return context.getText();
   }
 }
 
+// TODO(jmesserly): rename to Module.
 class Program extends Node {
   /// Script tag hash-bang, e.g. `#!/usr/bin/env node`
   final String scriptTag;
 
   /// Top-level statements in the program.
   final List<ModuleItem> body;
 
   /// The module's own name.
   ///
   /// This is not used in ES6, but is provided to allow module lowering.
@@ skipping 1548 matching lines @@
 
   final List<ModuleItem> body;
   Module(this.body, {this.name});
 
   accept(NodeVisitor visitor) => visitor.visitModule(this);
   void visitChildren(NodeVisitor visitor) {
     for (ModuleItem item in body) item.accept(visitor);
   }
   Module _clone() => new Module(body);
 }