Improved error messages from unittest.

pkg/unittest/lib/src/future_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.
 
 part of matcher;
 
 /**
  * Matches a [Future] that completes successfully with a value. Note that this
  * creates an asynchronous expectation. The call to `expect()` that includes
  * this will return immediately and execution will continue. Later, when the
@@ skipping 18 matching lines @@
  */
 Matcher completion(matcher, [String id = '']) =>
     new _Completes(wrapMatcher(matcher), id);
 
 class _Completes extends BaseMatcher {
   final Matcher _matcher;
   final String _id;
 
   const _Completes(this._matcher, this._id);
 
-  bool matches(item, MatchState matchState) {
+  bool matches(item, Map matchState) {
     if (item is! Future) return false;
     var done = wrapAsync((fn) => fn(), _id);
 
     item.then((value) {
       done(() { if (_matcher != null) expect(value, _matcher); });
     }, onError: (error) {
       var id = _id == '' ? '' : '${_id} ';
       var reason = 'Expected future ${id}to complete successfully, '
                    'but it failed with ${error}';
       var trace = getAttachedStackTrace(error);
@@ skipping 10 matching lines @@
 
   Description describe(Description description) {
     if (_matcher == null) {
       description.add('completes successfully');
     } else {
       description.add('completes to a value that ').addDescriptionOf(_matcher);
     }
     return description;
   }
 }