Update unittest to new package layout.

pkg/unittest/lib/src/core_matchers.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.
 
 
 /**
  * Returns a matcher that matches empty strings, maps or collections.
  */
 const Matcher isEmpty = const _Empty();
 
@@ skipping 606 matching lines @@
   final _matcher;
   final String _description;
 
   const _Predicate(this._matcher, this._description);
 
   bool matches(item, MatchState matchState) => _matcher(item);
 
   Description describe(Description description) =>
       description.add(_description);
 }