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

pkg/dartdoc/lib/src/markdown/inline_parser.dart

Index: pkg/dartdoc/lib/src/markdown/inline_parser.dart
diff --git a/pkg/dartdoc/lib/src/markdown/inline_parser.dart b/pkg/dartdoc/lib/src/markdown/inline_parser.dart
index a9588888c93489543e8f4171f5776720ee0875fa..554477387d706d197e80d6b0331e1358db142a9d 100644
--- a/pkg/dartdoc/lib/src/markdown/inline_parser.dart
+++ b/pkg/dartdoc/lib/src/markdown/inline_parser.dart
@@ -148,7 +148,7 @@ class InlineParser {
 }
 
 /// Represents one kind of markdown tag that can be parsed.
-class InlineSyntax {
+abstract class InlineSyntax {
   final RegExp pattern;
 
   InlineSyntax(String pattern)
@@ -168,7 +168,7 @@ class InlineSyntax {
     return false;
   }
 
-  abstract bool onMatch(InlineParser parser, Match match);
+  bool onMatch(InlineParser parser, Match match);
 }
 
 /// Matches stuff that should just be passed through as straight text.