Remove AbstractVisitor.

dart/lib/compiler/implementation/tree_validator.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.
 
 class TreeValidatorTask extends CompilerTask {
   TreeValidatorTask(Compiler compiler) : super(compiler);
 
   void validate(Node tree) {
     assert(check(tree));
   }
 
   bool check(Node tree) {
     List<InvalidNodeError> errors = [];
     void report(node, message) {
       final error = new InvalidNodeError(node, message);
       errors.add(error);
       compiler.reportWarning(node, message);
     };
     final validator = new ValidatorVisitor(report);
     tree.accept(new TraversingVisitor(validator));
 
     return errors.isEmpty();
   }
 }
 
-class ValidatorVisitor extends AbstractVisitor {
+class ValidatorVisitor extends Visitor {
   final Function reportInvalidNode;
 
   ValidatorVisitor(Function this.reportInvalidNode);
 
   expect(Node node, bool test, [message]) {
     if (!test) reportInvalidNode(node, message);
   }
 
   visitNode(Node node) {}
 
@@ skipping 22 matching lines @@
   final String message;
   InvalidNodeError(this.node, [this.message]);
 
   toString() {
     String nodeString = node.toDebugString();
     String result = 'invalid node: $nodeString';
     if (message !== null) result = '$result ($message)';
     return result;
   }
 }