Remove AbstractVisitor.

dart/lib/compiler/implementation/dart_backend/backend.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.
 
 const bool REMOVE_ASSERTS = false;
 
 class ElementAst {
   final Node ast;
   final TreeElements treeElements;
 
@@ skipping 406 matching lines @@
 
   log(String message) => compiler.log('[DartBackend] $message');
 }
 
 /**
  * Some elements are not recorded by resolver now,
  * for example, typedefs or classes which are only
  * used in signatures, as/is operators or in super clauses
  * (just to name a few).  Retraverse AST to pick those up.
  */
-class ReferencedElementCollector extends AbstractVisitor {
+class ReferencedElementCollector extends Visitor {
   final Compiler compiler;
   final Element rootElement;
   final TreeElements treeElements;
   final newTypedefElementCallback;
   final newClassElementCallback;
 
   ReferencedElementCollector(
       this.compiler,
       Element rootElement, this.treeElements,
       this.newTypedefElementCallback, this.newClassElementCallback)
@@ skipping 39 matching lines @@
 }
 
 compareElements(e0, e1) {
   int result = compareBy((e) => e.getLibrary().uri.toString())(e0, e1);
   if (result != 0) return result;
   return compareBy((e) => e.position().charOffset)(e0, e1);
 }
 
 List<Element> sortElements(Collection<Element> elements) =>
     sorted(elements, compareElements);