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

pkg/dartdoc/lib/src/markdown/block_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.
 
 /// The line contains only whitespace or is empty.
 const _RE_EMPTY = const RegExp(r'^([ \t]*)$');
 
 /// A series of `=` or `-` (on the next line) define setext-style headers.
 const _RE_SETEXT = const RegExp(r'^((=+)|(-+))$');
 
@@ skipping 62 matching lines @@
     return regex.firstMatch(current) != null;
   }
 
   /// Gets whether or not the current line matches the given pattern.
   bool matchesNext(RegExp regex) {
     if (next == null) return false;
     return regex.firstMatch(next) != null;
   }
 }
 
-class BlockSyntax {
+abstract class BlockSyntax {
   /// Gets the collection of built-in block parsers. To turn a series of lines
   /// into blocks, each of these will be tried in turn. Order matters here.
   static List<BlockSyntax> get syntaxes {
     // Lazy initialize.
     if (_syntaxes == null) {
       _syntaxes = [
           new EmptyBlockSyntax(),
           new BlockHtmlSyntax(),
           new SetextHeaderSyntax(),
           new HeaderSyntax(),
@@ skipping 13 matching lines @@
 
   /// Gets the regex used to identify the beginning of this block, if any.
   RegExp get pattern => null;
 
   bool get canEndBlock => true;
 
   bool canParse(BlockParser parser) {
     return pattern.firstMatch(parser.current) != null;
   }
 
-  abstract Node parse(BlockParser parser);
+  Node parse(BlockParser parser);
 
   List<String> parseChildLines(BlockParser parser) {
     // Grab all of the lines that form the blockquote, stripping off the ">".
     final childLines = <String>[];
 
     while (!parser.isDone) {
       final match = pattern.firstMatch(parser.current);
       if (match == null) break;
       childLines.add(match[1]);
       parser.advance();
@@ skipping 124 matching lines @@
 }
 
 class ListItem {
   bool forceBlock = false;
   final List<String> lines;
 
   ListItem(this.lines);
 }
 
 /// Base class for both ordered and unordered lists.
-class ListSyntax extends BlockSyntax {
+abstract class ListSyntax extends BlockSyntax {
   bool get canEndBlock => false;
 
-  abstract String get listTag;
+  String get listTag;
 
   Node parse(BlockParser parser) {
     final items = <ListItem>[];
     var childLines = <String>[];
 
     endItem() {
       if (childLines.length > 0) {
         items.add(new ListItem(childLines));
         childLines = <String>[];
       }
@@ skipping 151 matching lines @@
     while (!BlockSyntax.isAtBlockEnd(parser)) {
       childLines.add(parser.current);
       parser.advance();
     }
 
     final contents = parser.document.parseInline(
         Strings.join(childLines, '\n'));
     return new Element('p', contents);
   }
 }