Remove obsolete test.

tests/compiler/dart2js_extra/isolate2_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.
-// TODO(7769): Remove this test when
-// tests/isolate/isolate2_negative_test passes.
-
-library isolate2_negative_test;
-import 'dart:isolate';
-
-void entry(SendPort replyTo) {
-  throw "foo";  /// 01: runtime error
-  replyTo.send("done");
-}
-
-main() {
-  var port = new ReceivePort();
-  Isolate.spawn(entry, port.sendPort);
-  port.first;
-}