fix type annotation in _lookUpMemberInInterfaces

pkg/analyzer/lib/src/generated/element.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.element;
 
 import 'dart:collection';
 import 'dart:math' show min;
 
 import 'package:analyzer/src/generated/utilities_general.dart';
@@ skipping 7254 matching lines @@
       }
     }
     for (InterfaceType interfaceType in targetType.interfaces) {
       ExecutableElement member = _lookUpMemberInInterfaces(
           interfaceType, true, library, visitedInterfaces, getMember);
       if (member != null) {
         return member;
       }
     }
     for (InterfaceType mixinType in targetType.mixins.reversed) {
-      PropertyAccessorElement member = _lookUpMemberInInterfaces(
+      ExecutableElement member = _lookUpMemberInInterfaces(
           mixinType, true, library, visitedInterfaces, getMember);
       if (member != null) {
         return member;
       }
     }
     InterfaceType superclass = targetType.superclass;
     if (superclass == null) {
       return null;
     }
     return _lookUpMemberInInterfaces(
@@ skipping 3851 matching lines @@
 
   @override
   void visitElement(Element element) {
     int offset = element.nameOffset;
     if (offset != -1) {
       map[offset] = element;
     }
     super.visitElement(element);
   }
 }