Update unittest to new package layout.

pkg/unittest/lib/src/map_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 which matches maps containing the given [value].
  */
 Matcher containsValue(value) => new _ContainsValue(value);
 
 class _ContainsValue extends BaseMatcher {
@@ skipping 35 matching lines @@
           add(" doesn't contain key ").addDescriptionOf(_key);
     } else {
       mismatchDescription.add(' contains key ').addDescriptionOf(_key).
           add(' but with value ');
       _valueMatcher.describeMismatch(item[_key], mismatchDescription,
           matchState, verbose);
       return mismatchDescription;
     }
   }
 }