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

pkg/unittest/lib/src/map_matchers.dart

Index: pkg/unittest/lib/src/map_matchers.dart
diff --git a/pkg/unittest/lib/src/map_matchers.dart b/pkg/unittest/lib/src/map_matchers.dart
index eccc4ecfc9a6fdcfddce4461f8061597726b8b6f..72984f9903f77d70fea8c93033d38cd3c312ae5e 100644
--- a/pkg/unittest/lib/src/map_matchers.dart
+++ b/pkg/unittest/lib/src/map_matchers.dart
@@ -44,8 +44,8 @@ class _ContainsMapping extends BaseMatcher {
   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 ');