Fix type variables in closures.

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 351 matching lines @@
   bool get isCall => !isOperator && !isPropertyAccess;
   bool get isIndex =>
       isOperator && identical(selector.asOperator().source.stringValue, '[]');
   bool get isLogicalAnd =>
       isOperator && identical(selector.asOperator().source.stringValue, '&&');
   bool get isLogicalOr =>
       isOperator && identical(selector.asOperator().source.stringValue, '||');
   bool get isParameterCheck =>
       isOperator && identical(selector.asOperator().source.stringValue, '?');
 
+  bool get isIsCheck {
+    return isOperator
+        && identical(selector.asOperator().source.stringValue, 'is')
+        && arguments.head.asSend() == null;
+  }
+
   bool get isIsNotCheck {
     return isOperator
         && identical(selector.asOperator().source.stringValue, 'is')
         && arguments.head.asSend() != null;
   }
 
   TypeAnnotation get typeAnnotationFromIsCheck {
     assert(isOperator
         && identical(selector.asOperator().source.stringValue, 'is'));
     return isIsNotCheck
@@ skipping 1718 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);
 }