Improved error messages from unittest.

pkg/unittest/lib/src/basematcher.dart

Index: pkg/unittest/lib/src/basematcher.dart
===================================================================
--- pkg/unittest/lib/src/basematcher.dart	(revision 23884)
+++ pkg/unittest/lib/src/basematcher.dart	(working copy)
@@ -50,8 +50,11 @@
    * 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;
 }