Dart: Reorganize files

mojo/dart/unittests/embedder_tests/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_for_test_only/expect.dart';
 import 'package:mojo/core.dart';
@@ skipping 25 matching lines @@
 
   // The handle will be closed by the MojoHandle finalizer during GC, so the
   // attempt to close it again will fail.
   int result = handle.close();
   Expect.isTrue(result == MojoResult.kInvalidArgument);
 }
 
 main() {
   test();
 }