Rationalize Dart mojo and sky package structure

examples/dart/hello_world/hello/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.
 
 // To run this app:
 // mojo_shell mojo:hello
 
 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';
 
 class Hello extends Application {
   Hello.fromHandle(MojoHandle handle) : super.fromHandle(handle);
 
   void initialize(List<String> args, String url) {
     print("$url Hello");
 
     // We expect to find the world mojo application at the same
     // path as this application.
     var c = connectToApplication(url.replaceAll("hello", "world"));
 
     // A call to close() here would cause this app to go down before the "world"
     // app has a chance to come up. Instead, we wait to close this app until
     // the "world" app comes up, does its print, and closes its end of the
     // connection.
     c.onError = closeApplication;
   }
 
   Future closeApplication() async {
     await close();
     assert(MojoHandle.reportLeakedHandles());
   }
 }
 
 main(List args) {
   MojoHandle appHandle = new MojoHandle(args[0]);
   new Hello.fromHandle(appHandle);
 }