Improved error messages from unittest.

pkg/unittest/lib/src/basematcher.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;
 
 /**
  * MatchState is a simple wrapper around an arbitrary object.
  * [Matcher] [matches] methods can use this to store useful
  * information upon match failures, and this information will
@@ skipping 32 matching lines @@
    * by appending to [mismatchDescription].
    */
   Description describe(Description mismatchDescription);
 
   /**
    * Generates a description of the matcher failed for a particular
    * [item], by appending the description to [mismatchDescription].
    * It does not check whether the [item] fails the match, as it is
    * only called after a failed match. There may be additional info
    * about the mismatch in [matchState].
+   * The base matcher does not add anything as the actual value is
+   * typically sufficient, but matchers that can add valuable info
+   * should override this.
    */
   Description describeMismatch(item, Description mismatchDescription,
                                MatchState matchState, bool verbose) =>
-    mismatchDescription.add('was ').addDescriptionOf(item);
+    mismatchDescription;
 }