Move isolate tests to lib/isolate.

tests/corelib/int_from_environment_default_value_test.dart

-// Copyright (c) 2013, 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.
-
-import "dart:isolate";
-
-import "package:expect/expect.dart";
-
-void test(port) {
-  Expect.isNull(const int.fromEnvironment('NOT_FOUND'));
-  Expect.equals(12345, const int.fromEnvironment('NOT_FOUND',
-                                                 defaultValue: 12345));
-  if (port != null) port.send(null);
-}
-
-main() {
-  test(null);
-  var port = new ReceivePort();
-  Isolate.spawn(test, port.sendPort);
-  port.listen((_) => port.close());
-}