Initial --modules=es6 support

lib/src/js/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> {
   T visitProgram(Program node);
 
   T visitBlock(Block node);
@@ skipping 266 matching lines @@
     context.buffer.write('`');
     return context.getText();
   }
 }
 
 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<Statement> body;
+  final List<ModuleItem> body;
 
   Program(this.body, {this.scriptTag});
 
   accept(NodeVisitor visitor) => visitor.visitProgram(this);
   void visitChildren(NodeVisitor visitor) {
-    for (Statement statement in body) statement.accept(visitor);
+    for (ModuleItem statement in body) statement.accept(visitor);
   }
   Program _clone() => new Program(body);
 }
 
 abstract class Statement extends ModuleItem {
   Statement toStatement() => this;
   Statement toReturn() => new Block([this, new Return()]);
 }
 
 class Block extends Statement {
@@ skipping 1459 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);
 }