Rationalize Dart mojo and sky package structure

mojo/dart/testing/lib/expect.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /**
  * This library contains an Expect class with static methods that can be used
  * for simple unit-tests.
  */
 library expect;
 
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 bool _identical(a, b) => identical(a, b);
 
 typedef bool _CheckExceptionFn(exception);
 
 class ExpectException implements Exception {
   ExpectException(this.message);
   String toString() => message;
   String message;
 }