Clean up:

lib/src/ast.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.
 
-library markdown.ast;
+library markdown.src.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.
 abstract class Node {
   void accept(NodeVisitor visitor);
 }
 
 /// A named tag that can contain other nodes.
 class Element implements Node {
   final String tag;
   final List<Node> children;
   final Map<String, String> attributes;
 
   Element(this.tag, this.children) : attributes = <String, String>{};
 
   Element.empty(this.tag)
       : children = null,
-        attributes = <String, String>{};
+        attributes = {};
 
   Element.withTag(this.tag)
       : children = [],
-        attributes = <String, String>{};
+        attributes = {};
 
   Element.text(this.tag, String text)
       : children = [new Text(text)],
-        attributes = <String, String>{};
+        attributes = {};
 
   bool get isEmpty => children == null;
 
   void accept(NodeVisitor visitor) {
     if (visitor.visitElementBefore(this)) {
-      for (final child in children) child.accept(visitor);
+      for (var child in children) child.accept(visitor);
       visitor.visitElementAfter(this);
     }
   }
 }
 
 /// A plain text element.
 class Text implements Node {
   final String text;
   Text(this.text);
 
   void accept(NodeVisitor visitor) => visitor.visitText(this);
 }
 
 /// Visitor pattern for the AST. Renderers or other AST transformers should
 /// implement this.
 abstract class NodeVisitor {
   /// Called when a Text node has been reached.
   void visitText(Text text);
 
   /// Called when an Element has been reached, before its children have been
   /// visited. Return `false` to skip its children.
   bool visitElementBefore(Element element);
 
   /// Called when an Element has been reached, after its children have been
   /// visited. Will not be called if [visitElementBefore] returns `false`.
   void visitElementAfter(Element element);
 }