Type cast fix.

lib/unittest/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.
  */
 final Matcher isEmpty = const _Empty();
 
@@ skipping 86 matching lines @@
     if (actual is !Iterable) {
       return 'is not Iterable';
     }
     var expectedIterator = expected.iterator();
     var actualIterator = actual.iterator();
     var position = 0;
     String reason = null;
     while (reason == null) {
       if (expectedIterator.hasNext()) {
         if (actualIterator.hasNext()) {
-          reason = matcher(expectedIterator.next(),
+          Description r = matcher(expectedIterator.next(),
                            actualIterator.next(),
                            'mismatch at position ${position}',
                            depth);
+          if (r != null) reason = r.toString();

[srdjan, 2012/06/21 00:07:33]

Why guard it (r != null)?

(null).toString() should return a valid string.

           ++position;
         } else {
           reason = 'shorter than expected';
         }
       } else if (actualIterator.hasNext()) {
         reason = 'longer than expected';
       } else {
         return null;
       }
     }
@@ skipping 459 matching lines @@
   final _matcher;
   final String _description;
 
   const _Predicate(this._matcher, this._description);
 
   bool matches(item) => _matcher(item);
 
   Description describe(Description description) =>
       description.add(_description);
 }