Move isolate tests to lib/isolate.

tests/isolate/string_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 String.fromEnvironment('NOT_FOUND'));
   Expect.equals('x',
                 const String.fromEnvironment('NOT_FOUND', defaultValue: 'x'));
   if (port != null) port.send(null);
 }
 
 main() {
   test(null);
   var port = new ReceivePort();
   Isolate.spawn(test, port.sendPort);
   port.listen((_) => port.close());
 }