Rationalize Dart mojo and sky package structure

mojo/dart/test/handle_finalizer_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 'dart:typed_data';
 
-import 'package:mojo/dart/testing/expect.dart';
-import 'package:mojo/public/dart/core.dart';
+import 'package:testing/expect.dart';
+import 'package:mojo/core.dart';
 
 MojoHandle leakMojoHandle() {
   var pipe = new MojoMessagePipe();
   Expect.isNotNull(pipe);
 
   var endpoint = pipe.endpoints[0];
   Expect.isTrue(endpoint.handle.isValid);
 
   var eventStream = new MojoEventStream(endpoint.handle);
   // After making a MojoEventStream, the underlying mojo handle will have
@@ skipping 15 matching lines @@
 
   // The handle will be closed by the MojoHandle finalizer during GC, so the
   // attempt to close it again will fail.
   MojoResult result = handle.close();
   Expect.isTrue(result.isInvalidArgument);
 }
 
 main() {
   test();
 }