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

pkg/dartdoc/lib/src/markdown/inline_parser.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.
 
 /// Maintains the internal state needed to parse inline span elements in
 /// markdown.
 class InlineParser {
   static List<InlineSyntax> get syntaxes {
     // Lazy initialize.
     if (_syntaxes == null) {
@@ skipping 130 matching lines @@
     pos += length;
   }
 
   void consume(int length) {
     pos += length;
     start = pos;
   }
 }
 
 /// Represents one kind of markdown tag that can be parsed.
-class InlineSyntax {
+abstract class InlineSyntax {
   final RegExp pattern;
 
   InlineSyntax(String pattern)
     : pattern = new RegExp(pattern, multiLine: true);
 
   bool tryMatch(InlineParser parser) {
     final startMatch = pattern.firstMatch(parser.currentSource);
     if ((startMatch != null) && (startMatch.start == 0)) {
       // Write any existing plain text up to this point.
       parser.writeText();
 
       if (onMatch(parser, startMatch)) {
         parser.consume(startMatch[0].length);
       }
       return true;
     }
     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.
 class TextSyntax extends InlineSyntax {
   String substitute;
   TextSyntax(String pattern, {String sub})
     : super(pattern),
       substitute = sub;
 
   bool onMatch(InlineParser parser, Match match) {
@@ skipping 217 matching lines @@
       parser.consume(endMatch[0].length);
     } else {
       // Didn't close correctly so revert to text.
       parser.start = startPos;
       parser.advanceBy(endMatch[0].length);
     }
 
     return null;
   }
 }