Some minor small fixes to unittest

pkg/unittest/lib/src/core_matchers.dart

Index: pkg/unittest/lib/src/core_matchers.dart
diff --git a/pkg/unittest/lib/src/core_matchers.dart b/pkg/unittest/lib/src/core_matchers.dart
index 7855ed9207064b542d79a03d8c58daa75004d0b1..74e8366b6276f7cfb521cf002bee095833a825e9 100644
--- a/pkg/unittest/lib/src/core_matchers.dart
+++ b/pkg/unittest/lib/src/core_matchers.dart
@@ -284,10 +284,10 @@ class Throws extends BaseMatcher {
     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;