Dart: Reorganize files

mojo/dart/test/simple_handle_watcher_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';
-
-import 'package:mojo/core.dart' as core;
-
-main() {
-  var pipe = new core.MojoMessagePipe();
-  assert(pipe != null);
-
-  var endpoint = pipe.endpoints[0];
-  assert(endpoint.handle.isValid);
-
-  var eventSubscription = new core.MojoEventSubscription(endpoint.handle);
-  var completer = new Completer();
-  int numEvents = 0;
-
-  eventSubscription.subscribe((_) {
-    numEvents++;
-    eventSubscription.close().then((_) {
-      completer.complete(numEvents);
-    });
-  });
-  eventSubscription.enableWriteEvents();
-
-  completer.future.then((int numEvents) {
-    assert(numEvents == 1);
-  });
-}