Remove unused API.

dart/sdk/lib/_internal/compiler/implementation/elements/visitor.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.
 
 library visitor;
 
 import 'elements.dart';
 import '../ssa/ssa.dart'
     show InterceptedElement;
 import '../closure.dart'
     show ThisElement,
-         CheckVariableElement,
          BoxElement,
          BoxFieldElement,
          ClosureClassElement,
          ClosureFieldElement;
 
 abstract class ElementVisitor<R> {
   R visit(Element e) => e.accept(this);
 
   R visitElement(Element e);
   R visitErroneousElement(ErroneousElement e) => visitFunctionElement(e);
@@ skipping 14 matching lines @@
   R visitTypeDeclarationElement(TypeDeclarationElement e) => visitElement(e);
   R visitMixinApplicationElement(MixinApplicationElement e) {
     visitClassElement(e);
   }
   R visitVoidElement(VoidElement e) => visitElement(e);
   R visitLabelElement(LabelElement e) => visitElement(e);
   R visitTargetElement(TargetElement e) => visitElement(e);
   R visitTypeVariableElement(TypeVariableElement e) => visitElement(e);
   R visitInterceptedElement(InterceptedElement e) => visitElement(e);
   R visitThisElement(ThisElement e) => visitElement(e);
-  R visitCheckVariableElement(CheckVariableElement e ) => visitElement(e);
   R visitBoxElement(BoxElement e) => visitElement(e);
   R visitBoxFieldElement(BoxFieldElement e) => visitElement(e);
   R visitClosureClassElement(ClosureClassElement e) => visitClassElement(e);
   R visitClosureFieldElement(ClosureFieldElement e) => visitVariableElement(e);
 }