Rationalize Dart mojo and sky package structure

mojo/dart/test/simple_handle_watcher_test.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 'dart:isolate';
 
-import 'package:mojo/public/dart/core.dart' as core;
+import 'package:mojo/core.dart' as core;
 
 main() {
   var pipe = new core.MojoMessagePipe();
   assert(pipe != null);
 
   var endpoint = pipe.endpoints[0];
   assert(endpoint.handle.isValid);
 
   var eventStream = new core.MojoEventStream(endpoint.handle);
   var completer = new Completer();
   int numEvents = 0;
 
   eventStream.listen((_) {
     numEvents++;
     eventStream.close();
   }, onDone: () {
     completer.complete(numEvents);
   });
   eventStream.enableWriteEvents();
 
   completer.future.then((int numEvents) {
     assert(numEvents == 1);
   });
 }