Dart: Reorganize files

mojo/dart/apptests/test_apps/echo/lib/main.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 'package:mojo/application.dart';
 import 'package:mojo/bindings.dart';
 import 'package:mojo/core.dart';
 import 'package:_mojo_for_test_only/test/echo_service.mojom.dart';
@@ skipping 35 matching lines @@
 }
 
 main(List args) {
   MojoHandle appHandle = new MojoHandle(args[0]);
   String url = args[1];
   new EchoApplication.fromHandle(appHandle)
     ..onError = ((_) {
       MojoHandle.reportLeakedHandles();
     });
 }