Move isolate tests to lib/isolate.

tests/lib/async/stream_first_where_test.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 stream_controller_async_test;
 
 import "package:expect/expect.dart";
 import 'dart:async';
-import 'dart:isolate';
 import 'package:unittest/unittest.dart';
 import 'event_helper.dart';
 import 'stream_state_helper.dart';
 
 class A { const A(); }
 class B extends A { const B(); }
 
 main() {
   Events sentEvents = new Events()..close();
 
   // Make sure that firstWhere allows to return instances of types that are
   // different than the generic type of the stream.
   test("firstWhere with super class", () {
     StreamController c = new StreamController<B>();
     Future f = c.stream.firstWhere((x) => false, defaultValue: () => const A());
     f.then(expectAsync((v) { Expect.equals(const A(), v); }));
     sentEvents.replay(c);
   });
 }