Dart: Reorganize files

mojo/dart/unittests/embedder_tests/ping_pong_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 'dart:typed_data';
 
 import 'package:mojo/core.dart' as core;
 
@@ skipping 59 matching lines @@
         eventSubscription.enableReadEvents();
       } else if (core.MojoHandleSignals.isPeerClosed(mojoSignals)) {
         throw 'This end should close first.';
       } else {
         throw 'Unexpected event.';
       }
     });
     eventSubscription.enableWriteEvents();
   });
 }