Add pretty-printing to unittest and more thoroughly print values.

pkg/unittest/lib/src/map_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;
 
 /**
  * Returns a matcher which matches maps containing the given [value].
  */
 Matcher containsValue(value) => new _ContainsValue(value);
@@ skipping 26 matching lines @@
       _valueMatcher.matches(item[_key], matchState);
 
   Description describe(Description description) {
     return description.add('contains pair ').addDescriptionOf(_key).
           add(' => ').addDescriptionOf(_valueMatcher);
   }
 
   Description describeMismatch(item, Description mismatchDescription,
                                MatchState matchState, bool verbose) {
     if (!item.containsKey(_key)) {
-      return mismatchDescription.addDescriptionOf(item).
-          add(" doesn't contain key ").addDescriptionOf(_key);
+      return mismatchDescription.add(" doesn't contain key ")
+          .addDescriptionOf(_key);
     } else {
       mismatchDescription.add(' contains key ').addDescriptionOf(_key).
           add(' but with value ');
       _valueMatcher.describeMismatch(item[_key], mismatchDescription,
           matchState, verbose);
       return mismatchDescription;
     }
   }
 }