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

pkg/unittest/matcher.dart

Index: pkg/unittest/matcher.dart
diff --git a/pkg/unittest/matcher.dart b/pkg/unittest/matcher.dart
index d12418533e4aa5eb9d39c73a92d12087aee8fb6b..1c430f192fc645cc4901ac1d1e3a14ea5fa982aa 100644
--- a/pkg/unittest/matcher.dart
+++ b/pkg/unittest/matcher.dart
@@ -38,13 +38,13 @@ abstract class BaseMatcher implements Matcher {
    * [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