Some minor small fixes to unittest

pkg/unittest/lib/src/core_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 that matches empty strings, maps or collections.
  */
 const Matcher isEmpty = const _Empty();
@@ skipping 266 matching lines @@
 class Throws extends BaseMatcher {
   final Matcher _matcher;
 
   const Throws([Matcher matcher]) :
     this._matcher = matcher;
 
   bool matches(item, MatchState matchState) {
     if (item is Future) {
       // Queue up an asynchronous expectation that validates when the future
       // completes.
-      item.then((value) {
+      item.then(wrapAsync((value) {
         expect(false, isTrue, reason:
             "Expected future to fail, but succeeded with '$value'.");
-      });
+      }));
 
       item.catchError((e) {
         var reason;
         if (e.stackTrace != null) {
           var stackTrace = e.stackTrace.toString();
           stackTrace = "  ${stackTrace.replaceAll("\n", "\n  ")}";
           reason = "Actual exception trace:\n$stackTrace";
         }
         expect(e.error, _matcher, reason: reason);
       });
@@ skipping 376 matching lines @@
       description.add(_featureDescription).add(' ').addDescriptionOf(_matcher);
 
   Description describeMismatch(item, Description mismatchDescription,
                                MatchState matchState, bool verbose) {
     mismatchDescription.add(_featureName).add(' ');
     _matcher.describeMismatch(matchState.state['feature'], mismatchDescription,
         matchState.state['innerState'], verbose);
     return mismatchDescription;
   }
 }