Improve double implementation.

dart/frog/leg/resolver.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.
 
 interface TreeElements {
   Element operator[](Node node);
   Selector getSelector(Send send);
 }
 
 class TreeElementMapping implements TreeElements {
@@ skipping 354 matching lines @@
 
 class CommonResolverVisitor<R> extends AbstractVisitor<Element> {
   final Compiler compiler;
 
   CommonResolverVisitor(Compiler this.compiler);
 
   R visitNode(Node node) {
     cancel(node, 'internal error');
   }
 
+  R visitEmptyStatement(Node node) => null;
+
   /** Convenience method for visiting nodes that may be null. */
   R visit(Node node) => (node == null) ? null : node.accept(this);
 
   void error(Node node, MessageKind kind, [arguments = const []]) {
     ResolutionError error  = new ResolutionError(kind, arguments);
     compiler.reportError(node, error);
   }
 
   void warning(Node node, MessageKind kind, [arguments = const []]) {
     ResolutionWarning warning  = new ResolutionWarning(kind, arguments);
@@ skipping 970 matching lines @@
 class TopScope extends Scope {
   LibraryElement get library() => element;
 
   TopScope(LibraryElement library) : super(null, library);
   Element lookup(SourceString name) => library.find(name);
 
   Element add(Element element) {
     throw "Cannot add an element in the top scope";
   }
 }