Add missing newline after Actual value in fail message.

pkg/unittest/lib/src/map_matchers.dart

Index: pkg/unittest/lib/src/map_matchers.dart
===================================================================
--- pkg/unittest/lib/src/map_matchers.dart	(revision 24669)
+++ pkg/unittest/lib/src/map_matchers.dart	(working copy)
@@ -9,7 +9,7 @@
  */
 Matcher containsValue(value) => new _ContainsValue(value);
 
-class _ContainsValue extends BaseMatcher {
+class _ContainsValue extends Matcher {
   final _value;
 
   const _ContainsValue(this._value);
@@ -26,7 +26,7 @@
 Matcher containsPair(key, value) =>
     new _ContainsMapping(key, wrapMatcher(value));
 
-class _ContainsMapping extends BaseMatcher {
+class _ContainsMapping extends Matcher {
   final _key;
   final Matcher _valueMatcher;