Remove AbstractVisitor.

dart/lib/compiler/implementation/ssa/nodes.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.
 
 abstract class HVisitor<R> {
   R visitAdd(HAdd node);
   R visitBailoutTarget(HBailoutTarget node);
   R visitBitAnd(HBitAnd node);
   R visitBitNot(HBitNot node);
   R visitBitOr(HBitOr node);
@@ skipping 2761 matching lines @@
                                     [this.isContinue = false])
       : this.labels = const<LabelElement>[];
 
   HBasicBlock get start => body.start;
   HBasicBlock get end => body.end;
 
   bool accept(HStatementInformationVisitor visitor) =>
     visitor.visitLabeledBlockInfo(this);
 }
 
-class LoopTypeVisitor extends AbstractVisitor {
+class LoopTypeVisitor extends Visitor {
   const LoopTypeVisitor();
   int visitNode(Node node) => HLoopBlockInformation.NOT_A_LOOP;
   int visitWhile(While node) => HLoopBlockInformation.WHILE_LOOP;
   int visitFor(For node) => HLoopBlockInformation.FOR_LOOP;
   int visitDoWhile(DoWhile node) => HLoopBlockInformation.DO_WHILE_LOOP;
   int visitForIn(ForIn node) => HLoopBlockInformation.FOR_IN_LOOP;
 }
 
 class HLoopBlockInformation implements HStatementInformation {
   static const int WHILE_LOOP = 0;
@@ skipping 124 matching lines @@
   HBasicBlock get start => expression.start;
   HBasicBlock get end {
     // We don't create a switch block if there are no cases.
     assert(!statements.isEmpty());
     return statements.last().end;
   }
 
   bool accept(HStatementInformationVisitor visitor) =>
       visitor.visitSwitchInfo(this);
 }