Improved the way we generate mismatch descriptions.

lib/unittest/future_matchers.dart

Index: lib/unittest/future_matchers.dart
===================================================================
--- lib/unittest/future_matchers.dart	(revision 9956)
+++ lib/unittest/future_matchers.dart	(working copy)
@@ -29,7 +29,7 @@
 
   const _Completes(this._matcher);
 
-  bool matches(item) {
+  bool matches(item, MatchState matchState) {
     if (item is! Future) return false;
 
     item.onComplete(expectAsync1((future) {
@@ -56,4 +56,4 @@
     }
     return description;
   }
-}
+}