Move isolate tests to lib/isolate.

tests/corelib/bool_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.isFalse(const bool.fromEnvironment('NOT_FOUND'));
-  Expect.isTrue(const bool.fromEnvironment('NOT_FOUND', defaultValue: true));
-  Expect.isFalse(const bool.fromEnvironment('NOT_FOUND', defaultValue: false));
-  Expect.isNull(const bool.fromEnvironment('NOT_FOUND', defaultValue: null));
-  if (port != null) port.send(null);
-}
-
-main() {
-  test(null);
-  var port = new ReceivePort();
-  Isolate.spawn(test, port.sendPort);
-  port.listen((_) => port.close());
-}