Dart: Reorganize files

mojo/dart/test/isolate_test.dart

-// Copyright 2014 The Chromium Authors. 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:async';
-import 'dart:isolate';
-
-void foo(SendPort sp) {
-  var rp = new ReceivePort();
-  sp.send(rp.sendPort);
-  rp.listen((msg) {
-    if ((msg is String) && (msg == "Hello, world!")) {
-      print("Hello, world!");
-      rp.close();
-    }
-  });
-}
-
-main() {
-  var rp = new ReceivePort();
-  Isolate.spawn(foo, rp.sendPort).then((isolate) {
-    var sp = null;
-    rp.listen((msg) {
-      if (msg is SendPort) {
-        sp = msg;
-        sp.send("Hello, world!");
-        rp.close();
-      }
-    });
-  });
-}