Remove AbstractVisitor.

dart/lib/compiler/implementation/dart_backend/placeholder_collector.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.
 
 class LocalPlaceholder {
   final String identifier;
   final Set<Node> nodes;
   LocalPlaceholder(this.identifier) : nodes = new Set<Node>();
   int hashCode() => identifier.hashCode();
   String toString() =>
@@ skipping 119 matching lines @@
       // Hack: putting null into map overrides receiver of original node.
       collector.makeNullPlaceholder(node.receiver);
     }
   }
 
   internalError(String reason, [Node node]) {
     collector.internalError(reason, node);
   }
 }
 
-class PlaceholderCollector extends AbstractVisitor {
+class PlaceholderCollector extends Visitor {
   final Compiler compiler;
   final Set<String> fixedMemberNames; // member names which cannot be renamed.
   final Map<Element, ElementAst> elementAsts;
   final Set<Node> nullNodes;  // Nodes that should not be in output.
   final Set<Identifier> unresolvedNodes;
   final Map<Element, Set<Node>> elementNodes;
   final Map<FunctionElement, FunctionScope> functionScopes;
   final Map<LibraryElement, Set<Identifier>> privateNodes;
   final List<DeclarationTypePlaceholder> declarationTypePlaceholders;
   final Map<String, Set<Identifier>> memberPlaceholders;
@@ skipping 452 matching lines @@
 
   visitBlock(Block node) {
     for (Node statement in node.statements.nodes) {
       if (statement is VariableDefinitions) {
         makeVarDeclarationTypePlaceholder(statement);
       }
     }
     node.visitChildren(this);
   }
 }