Abstract is now required on abstract classes so adding that.

pkg/unittest/matcher.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.
 
 /**
  * MatchState is a simple wrapper around an arbitrary object.
  * [Matcher] [matches] methods can use this to store useful
  * information upon match failures, and this information will
  * be passed to [describeMismatch]. Each [Matcher] is responsible
  * for its own use of this state, so the state created by [matches]
  * should be consistent with that expected by [describeMismatch] in
  * the same [Matcher] class, but can vary between classes. The inner
  * state, if set, will typically be a [Map] with a number of key-value
  * pairs containing relevant state information.
  */
 class MatchState {
   var state = null;
 
   MatchState([this.state]);
 }
 
 /**
  * BaseMatcher is the base class for all matchers. To implement a new
  * matcher, either add a class that implements from IMatcher or
  * a class that inherits from Matcher. Inheriting from Matcher has
  * the benefit that a default implementation of describeMismatch will
  * be provided.
  */
-class BaseMatcher implements Matcher {
+abstract class BaseMatcher implements Matcher {
   const BaseMatcher();
 
   /**
    * Tests the matcher against a given [item]
    * and return true if the match succeeds; false otherwise.
    * [matchState] may be used to return additional info for
    * the use of [describeMismatch].
    */
   abstract bool matches(item, MatchState matchState);
 
   /**
    * Creates a textual description of a matcher,
    * by appending to [mismatchDescription].
    */
   abstract Description describe(Description mismatchDescription);
 
   /**
    * Generates a description of the matcher failed for a particular
    * [item], by appending the description to [mismatchDescription].
    * It does not check whether the [item] fails the match, as it is
    * only called after a failed match. There may be additional info
    * about the mismatch in [matchState].
    */
   Description describeMismatch(item, Description mismatchDescription,
                                MatchState matchState, bool verbose) =>
     mismatchDescription.add('was ').addDescriptionOf(item);
 }