Using 'interface' and 'abstract' for methods should produce error

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]
@@ skipping 20 matching lines @@
  */
 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);
+  bool matches(item, MatchState matchState);
 
   /**
    * Creates a textual description of a matcher,
    * by appending to [mismatchDescription].
    */
-  abstract Description describe(Description mismatchDescription);
+  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);
 }