Abstract is now required on abstract classes so adding that.

pkg/unittest/collection_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.
 
 /**
  * Returns a matcher which matches [Collection]s in which all elements
  * match the given [matcher].
  */
 Matcher everyElement(matcher) => new _EveryElement(wrapMatcher(matcher));
 
@@ skipping 165 matching lines @@
 
   Description describeMismatch(item, Description mismatchDescription,
                                MatchState matchState, bool verbose) =>
       mismatchDescription.add(_test(item));
 }
 
 /**
  * Collection matchers match against [Collection]s. We add this intermediate
  * class to give better mismatch error messages than the base Matcher class.
  */
-/* abstract */ class _CollectionMatcher extends BaseMatcher {
+abstract class _CollectionMatcher extends BaseMatcher {
   const _CollectionMatcher();
   Description describeMismatch(item, Description mismatchDescription,
                                MatchState matchState, bool verbose) {
     if (item is !Collection) {
       return mismatchDescription.
           addDescriptionOf(item).
           add(' not a collection');
     } else {
       return super.describeMismatch(item, mismatchDescription, matchState,
         verbose);
     }
   }
 }