Dart: Reorganize files

mojo/dart/unittests/embedder_tests/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_for_test_only/expect.dart';
 import 'package:mojo/bindings.dart' as bindings;
 import 'package:mojo/core.dart' as core;
 
@@ skipping 726 matching lines @@
   Expect.equals(str, result.string);
 }
 
 main() {
   testAlign();
   testBar();
   testFoo();
   testNamedRegion();
   testUtf8();
 }