Changed interfaces to abstract classes where possible in {lib,samples,pkg}

samples/markdown/ast.dart

 // Copyright (c) 2011, 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.
 
 /// Base class for any AST item. Roughly corresponds to Node in the DOM. Will
 /// be either an Element or Text.
-interface Node {
+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)
@@ skipping 24 matching lines @@
 /// 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.
-interface NodeVisitor {
+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);
 }