Rationalize Dart mojo and sky package structure

examples/dart/device_info/main.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.
 
 import 'dart:async';
 
-import 'package:mojo/public/dart/application.dart';
-import 'package:mojo/public/dart/bindings.dart';
-import 'package:mojo/public/dart/core.dart';
+import 'package:mojo/application.dart';
+import 'package:mojo/bindings.dart';
+import 'package:mojo/core.dart';
 import 'package:mojom/mojo/device_info.mojom.dart';
 
 class DeviceInfoApp extends Application {
   final DeviceInfoProxy _deviceInfo = new DeviceInfoProxy.unbound();
 
   DeviceInfoApp.fromHandle(MojoHandle handle) : super.fromHandle(handle);
 
   Future initialize(List<String> args, String url) async {
     connectToService("mojo:device_info", _deviceInfo);
     print(await _deviceInfo.ptr.getDeviceType());
     _deviceInfo.close();
     close();
   }
 }
 
 main(List args) {
   MojoHandle appHandle = new MojoHandle(args[0]);
   new DeviceInfoApp.fromHandle(appHandle);
 }