Rationalize Dart mojo and sky package structure

mojo/dart/test/core_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: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';
 
 invalidHandleTest() {
   MojoHandle invalidHandle = new MojoHandle(MojoHandle.INVALID);
 
   // Close.
   MojoResult result = invalidHandle.close();
   Expect.isTrue(result.isInvalidArgument);
 
   // Wait.
   MojoWaitResult mwr =
@@ skipping 273 matching lines @@
   duplicate.close();
   duplicate = null;
 }
 
 main() {
   invalidHandleTest();
   basicMessagePipeTest();
   basicDataPipeTest();
   basicSharedBufferTest();
 }