Avoid error about instantiating abstract class AsynchronousTestCase.

tests/isolate/src/TestFramework.dart

 // Copyright (c) 2011, 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("TestFramework");
 #import("dart:coreimpl");
 
 
 typedef void AsynchronousTestFunction(TestExpectation check);
 
@@ skipping 258 matching lines @@
     }
   }
 
   void removeRunning() {
     if (--running == 0) {
       keepalive.toSendPort().send(null, null);
       keepalive = null;
     }
   }
 
+  // AsynchronousTestCase.run() calls variable test, not function performTest.
+  void performTest() {
+    Expect.fail('performTest called in AsynchronousTestCase');
+  }
+
   AsynchronousTestFunction test;
 
   static int running = 0;
   static ReceivePort keepalive = null;
 
 }
 
 // TODO(mattsh) - remove this once we have migrated the rpc system
 class TestPromise<T> extends PromiseImpl<T> {
 
   TestPromise(this.expect) : super();
 
   void addCompleteHandler(void completeHandler(T result)) {
     super.addCompleteHandler(expect.runs1((T result) {
       completeHandler(result);
     }));
   }
 
   final TestExpectation expect;
 
 }