Fix a hint in resolver.dart.

pkg/analyzer/lib/src/generated/resolver.dart

 // Copyright (c) 2014, 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 engine.resolver;
 
 import "dart:math" as math;
 import 'dart:collection';
 
 import 'package:analyzer/src/generated/utilities_collection.dart';
@@ skipping 13969 matching lines @@
   }
 
   @override
   Object visitClassDeclaration(ClassDeclaration node) {
     _hasReferenceToSuper = false;
     super.visitClassDeclaration(node);
     ClassElementImpl classElement = _getClassElement(node.name);
     if (classElement != null)  {
       classElement.hasReferenceToSuper = _hasReferenceToSuper;
     }
+    return null;
   }
 
   @override
   void visitClassDeclarationInScope(ClassDeclaration node) {
     super.visitClassDeclarationInScope(node);
     ExtendsClause extendsClause = node.extendsClause;
     WithClause withClause = node.withClause;
     ImplementsClause implementsClause = node.implementsClause;
     ClassElementImpl classElement = _getClassElement(node.name);
     InterfaceType superclassType = null;
@@ skipping 1724 matching lines @@
    * library.
    */
   final HashSet<String> members = new HashSet<String>();
 
   /**
    * Names of resolved or unresolved class members that are read in the
    * library.
    */
   final HashSet<String> readMembers = new HashSet<String>();
 }