Rationalize Dart mojo and sky package structure

mojo/dart/embedder/io/mojo_patch.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:_mojo/public/dart/application.dart';
-import 'dart:_mojo/public/dart/bindings.dart';
-import 'dart:_mojo/public/dart/core.dart';
+import 'dart:_mojo/application.dart';
+import 'dart:_mojo/bindings.dart';
+import 'dart:_mojo/core.dart';
 import 'dart:_mojom/mojo/net_address.mojom.dart';
 import 'dart:_mojom/mojo/network_error.mojom.dart';
 import 'dart:_mojom/mojo/network_service.mojom.dart';
 import 'dart:_mojom/mojo/tcp_bound_socket.mojom.dart';
 import 'dart:_mojom/mojo/tcp_connected_socket.mojom.dart';
 import 'dart:_mojom/mojo/tcp_server_socket.mojom.dart';
 
 //
 // Mojo objects and helper functions used by the 'dart:io' library.
 //
@@ skipping 152 matching lines @@
   }
 
   static _throwOnError(NetworkError error) {
     if (_okay(error)) {
       return;
     }
     throw new OSError(error.description, error.code);
   }
 }