Update unittest to new package layout.

pkg/unittest/lib/src/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 179 matching lines @@
     if (item is !Collection) {
       return mismatchDescription.
           addDescriptionOf(item).
           add(' not a collection');
     } else {
       return super.describeMismatch(item, mismatchDescription, matchState,
         verbose);
     }
   }
 }