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

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

Index: pkg/dartdoc/lib/src/markdown/block_parser.dart
diff --git a/pkg/dartdoc/lib/src/markdown/block_parser.dart b/pkg/dartdoc/lib/src/markdown/block_parser.dart
index 79d9e69f13bf23b24e8a0d94da2f6b30503b8b25..9d09f38d629272c2df5004d55c1d238f7534922d 100644
--- a/pkg/dartdoc/lib/src/markdown/block_parser.dart
+++ b/pkg/dartdoc/lib/src/markdown/block_parser.dart
@@ -80,7 +80,7 @@ class BlockParser {
   }
 }
 
-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 {
@@ -114,7 +114,7 @@ class BlockSyntax {
     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 ">".
@@ -259,10 +259,10 @@ class ListItem {
 }
 
 /// 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>[];