Implement is-checks for closures.

sdk/lib/_internal/compiler/implementation/elements/elements.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.
 
 library elements;
 
 import 'dart:uri';
 
 // TODO(ahe): Rename prefix to 'api' when VM bug is fixed.
 import '../../compiler.dart' as api_e;
@@ skipping 1471 matching lines @@
 
   /**
    * Lookup local members in the class. This will ignore constructors.
    */
   Element lookupLocalMember(SourceString memberName) {
     var result = localLookup(memberName);
     if (result != null && result.isConstructor()) return null;
     return result;
   }
 
+  /// Lookup a synthetic element created by the backend.
+  Element lookupBackendMember(SourceString memberName) {
+    for (Element element in backendMembers) {
+      if (element.name == memberName) {
+        return element;
+      }
+    }
+  }
+
   /**
    * Lookup super members for the class. This will ignore constructors.
    */
   Element lookupSuperMember(SourceString memberName) {
     return lookupSuperMemberInLibrary(memberName, getLibrary());
   }
 
   /**
    * Lookup super members for the class that is accessible in [library].
    * This will ignore constructors.
@@ skipping 630 matching lines @@
 
   MetadataAnnotation ensureResolved(Compiler compiler) {
     if (resolutionState == STATE_NOT_STARTED) {
       compiler.resolver.resolveMetadataAnnotation(this);
     }
     return this;
   }
 
   String toString() => 'MetadataAnnotation($value, $resolutionState)';
 }