Clean up isolate tests.

tests/isolate/isolate2_negative_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 testing that exceptions in other isolates bring down
-// the program.
-
-library isolate2_negative_test;
-import 'dart:isolate';
-import "package:async_helper/async_helper.dart";
-
-void entry(msg) {
-  throw "foo";
-}
-
-main() {
-  // We start an asynchronous operation, but since we don't expect to get
-  // anything back except an exception there is no asyncEnd().
-  // If the exception is not thrown this test will timeout.
-  asyncStart();
-  Isolate.spawn(entry, null);
-}