Improved error messages from unittest.

pkg/unittest/test/test_utils.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.
 
 library test_utils;
 
 import 'package:unittest/unittest.dart';
 
 import 'dart:async';
 
@@ skipping 51 matching lines @@
     afterTest();
   }
 }
 
 doesNotThrow() {}
 doesThrow() { throw 'X'; }
 
 class PrefixMatcher extends BaseMatcher {
   final String _prefix;
   const PrefixMatcher(this._prefix);
-  bool matches(item, MatchState matchState) {
+  bool matches(item, Map matchState) {
     return item is String &&
         (collapseWhitespace(item)).startsWith(collapseWhitespace(_prefix));
   }
 
   Description describe(Description description) =>
     description.add('a string starting with ').
         addDescriptionOf(collapseWhitespace(_prefix)).
         add(' ignoring whitespace');
 }