Update unittest to new package layout.

pkg/unittest/lib/src/string_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 if the match argument is a string and
  * is equal to [value] when compared case-insensitively.
  */
 Matcher equalsIgnoringCase(String value) => new _IsEqualIgnoringCase(value);
 
@@ skipping 180 matching lines @@
     if (!(item is String)) {
       return mismatchDescription.
           addDescriptionOf(item).
           add(' not a string');
     } else {
       return super.describeMismatch(item, mismatchDescription,
           matchState, verbose);
     }
   }
 }