Move isolate tests to lib/isolate.

tests/isolate/timer_isolate_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.
 
 library multiple_timer_test;
 
 import 'dart:isolate';
 import 'dart:async';
 import 'package:unittest/unittest.dart';
 
@@ skipping 20 matching lines @@
     port.first.then(expectAsync((msg) {
       expect("timer_fired", msg);
       int endTime = (new DateTime.now()).millisecondsSinceEpoch;
       expect(endTime - startTime + safetyMargin, greaterThanOrEqualTo(TIMEOUT.inMilliseconds));
     }));
 
     startTime = (new DateTime.now()).millisecondsSinceEpoch;
     var remote = Isolate.spawn(createTimer, port.sendPort);
   });
 }