fix implicit_scope_test and builtin_identifier_test

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 260 matching lines @@
   Program _clone() => new Program(body);
 }
 
 abstract class Statement extends Node {
   Statement toStatement() => this;
   Statement toReturn() => new Block([this, new Return()]);
 }
 
 class Block extends Statement {
   final List<Statement> statements;
-  Block(this.statements) {
+
+  /// True to preserve this [Block] for scoping reasons.
+  final bool isScope;
+
+  Block(this.statements, {this.isScope: false}) {
     assert(!statements.any((s) => s is! Statement));
   }
   Block.empty() : this.statements = <Statement>[];
 
   accept(NodeVisitor visitor) => visitor.visitBlock(this);
   void visitChildren(NodeVisitor visitor) {
     for (Statement statement in statements) statement.accept(visitor);
   }
   Block _clone() => new Block(statements);
 }
@@ skipping 1271 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);
 }