Rationalize Dart mojo and sky package structure

mojo/dart/test/codec_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/bindings.dart' as bindings;
-import 'package:mojo/public/dart/core.dart' as core;
+import 'package:testing/expect.dart';
+import 'package:mojo/bindings.dart' as bindings;
+import 'package:mojo/core.dart' as core;
 
 class Bar extends bindings.Struct {
   static const int kStructSize = 16;
   static const bindings.StructDataHeader kDefaultStructInfo =
       const bindings.StructDataHeader(kStructSize, 4);
 
   static final int Type_VERTICAL = 1;
   static final int Type_HORIZONTAL = Type_VERTICAL + 1;
   static final int Type_BOTH = Type_HORIZONTAL + 1;
   static final int Type_INVALID = Type_BOTH + 1;
@@ skipping 717 matching lines @@
   Expect.equals(str, result.string);
 }
 
 main() {
   testAlign();
   testBar();
   testFoo();
   testNamedRegion();
   testUtf8();
 }