pkg/docgen: update markdown to 0.5.1

third_party/pkg/markdown/lib/src/ast.dart

Index: third_party/pkg/markdown/lib/src/ast.dart
diff --git a/third_party/pkg/markdown/lib/src/markdown/ast.dart b/third_party/pkg/markdown/lib/src/ast.dart
similarity index 96%
rename from third_party/pkg/markdown/lib/src/markdown/ast.dart
rename to third_party/pkg/markdown/lib/src/ast.dart
index c966cea00cfd822d5d7ef9bad0bcb915abbb20e4..befd75e6bf733737d7a733efe2a3c9813aec99c8 100644
--- a/third_party/pkg/markdown/lib/src/markdown/ast.dart
+++ b/third_party/pkg/markdown/lib/src/ast.dart
@@ -2,7 +2,9 @@
 // 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.
 
-part of markdown;
+library markdown.ast;
+
+typedef Node Resolver(String name);
 
 /// Base class for any AST item. Roughly corresponds to Node in the DOM. Will
 /// be either an Element or Text.