cleanup: js_codegen now returns a js_ast.Program

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 1358 matching lines @@
 class Await extends Expression {
   /** The awaited expression. */
   final Expression expression;
 
   Await(this.expression);
 
   int get precedenceLevel => UNARY;
   accept(NodeVisitor visitor) => visitor.visitAwait(this);
   void visitChildren(NodeVisitor visitor) => expression.accept(visitor);
   Await _clone() => new Await(expression);
-
 }
 
 /**
  * A comment.
  *
  * Extends [Statement] so we can add comments before statements in
  * [Block] and [Program].
  */
 class Comment extends Statement {
   final String comment;
@@ skipping 17 matching lines @@
   final Expression expression;
 
   CommentExpression(this.comment, this.expression);
 
   int get precedenceLevel => PRIMARY;
   accept(NodeVisitor visitor) => visitor.visitCommentExpression(this);
   CommentExpression _clone() => new CommentExpression(comment, expression);
 
   void visitChildren(NodeVisitor visitor) => expression.accept(visitor);
 }