Abstract is now required on abstract classes so adding that.

pkg/unittest/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.
 
 
 /**
  * Returns a matcher that matches empty strings, maps or collections.
  */
 const Matcher isEmpty = const _Empty();
 
@@ skipping 375 matching lines @@
  *
  * bool _isException(x) => x is Exception;
  * final Matcher isException = const _Predicate(_isException, "Exception");
  * final Matcher throwsException = const _Throws(isException);
  *
  * But currently using static functions in const expressions is not supported.
  * For now the only solution for all platforms seems to be separate classes
  * for each exception type.
  */
 
-/* abstract */ class ExceptionMatcher extends BaseMatcher {
+abstract class ExceptionMatcher extends BaseMatcher {
   final String _name;
   const ExceptionMatcher(this._name);
   Description describe(Description description) =>
       description.add(_name);
 }
 
 /** A matcher for FormatExceptions. */
 const isFormatException = const _FormatException();
 
 /** A matcher for functions that throw FormatException */
@@ skipping 211 matching lines @@
   final _matcher;
   final String _description;
 
   const _Predicate(this._matcher, this._description);
 
   bool matches(item, MatchState matchState) => _matcher(item);
 
   Description describe(Description description) =>
       description.add(_description);
 }