Change NotImplementedException to UnimplementedError.

runtime/tests/vm/dart/isolate_mirror_remote_test.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.
 //
 // Dart test program for checking implemention of MirrorSystem when
 // inspecting a remote isolate.
 
 #library('isolate_mirror_local_test');
 
 #import('dart:isolate');
 #import('dart:mirrors');
 
 void isolateMain() {
   port.receive(
       (msg, replyPort) {
         Expect.fail('Received unexpected message $msg in remote isolate.');
       });
 }
 
 void testMirrorSystem(MirrorSystem mirror) {
   Expect.fail('Should not reach here.  Remote isolates not implemented.');
 }
 
 void main() {
   SendPort sp = spawnFunction(isolateMain);
   try {
     mirrorSystemOf(sp).then(testMirrorSystem);
     Expect.fail('Should not reach here.  Remote isolates not implemented.');
   } catch (exception) {
-    Expect.isTrue(exception is NotImplementedException);
+    Expect.isTrue(exception is UnimplementedError);
   }
 }