Update unittest to new package layout.

pkg/unittest/lib/src/operator_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.
 
 /**
  * This returns a matcher that inverts [matcher] to its logical negation.
  */
 Matcher isNot(matcher) => new _IsNot(wrapMatcher(matcher));
 
 class _IsNot extends BaseMatcher {
@@ skipping 159 matching lines @@
          return true;
        }
      }
      return false;
   }
 
   Description describe(Description description) =>
     description.addAll('(', ' or ', ')', _matchers);
 }