Dart: Reorganize files

mojo/dart/apptests/dart_apptests/lib/src/echo_apptests.dart

 // Copyright 2015 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.
 
 library echo_apptests;
 
 import 'dart:async';
 
 import 'package:mojo_apptest/apptest.dart';
 import 'package:mojo/application.dart';
@@ skipping 204 matching lines @@
                         .then((_) {
         fail('This should be unreachable');
       }, onError: (e) {
         expect(e is ProxyError, isTrue);
       });
 
       return Future.wait([f1, f2]);
     });
   });
 }