Fix crashes in scanner.

dart/sdk/lib/_internal/compiler/implementation/tree/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 tree;
 
 abstract class Visitor<R> {
   const Visitor();
 
   R visitNode(Node node);
@@ skipping 297 matching lines @@
   Statement();
 
   Statement asStatement() => this;
 
   // TODO(ahe): make class abstract instead of adding an abstract method.
   accept(Visitor visitor);
 
   bool isValidBreakTarget() => true;
 }
 
+/// Errorneous expression that behaves as a literal integer (0).
+class ErrorExpression extends LiteralInt {
+  ErrorExpression(token)
+      : super(token, null);
+
+  ErrorExpression asErrorExpression() => this;
+
+  int get value => 0;
+}
+
 /**
  * A message send aka method invocation. In Dart, most operations can
  * (and should) be considered as message sends. Getters and setters
  * are just methods with a special syntax. Consequently, we model
  * property access, assignment, operators, and method calls with this
  * one node.
  */
 class Send extends Expression {
   final Node receiver;
   final Node selector;
@@ skipping 1752 matching lines @@
  * argument).
  *
  * TODO(ahe): This method is controversial, the team needs to discuss
  * if top-level methods are acceptable and what naming conventions to
  * use.
  */
 initializerDo(Node node, f(Node node)) {
   SendSet send = node.asSendSet();
   if (send != null) return f(send.arguments.head);
 }