Don't use ClassElementImpl - does not work well with summaries.

pkg/analyzer/lib/src/dart/element/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 analyzer.src.dart.element.element;
 
 import 'dart:collection';
 import 'dart:math' show min;
 
 import 'package:analyzer/dart/ast/ast.dart';
@@ skipping 190 matching lines @@
       assert(false);
       return false;
     }
     // Find the nearest class in the supertype chain that is not a mixin
     // application.
     ClassElement nearestNonMixinClass = supertype.element;
     if (nearestNonMixinClass.isMixinApplication) {
       // Use a list to keep track of the classes we've seen, so that we won't
       // go into an infinite loop in the event of a non-trivial loop in the
       // class hierarchy.
-      List<ClassElementImpl> classesSeen = <ClassElementImpl>[this];
+      List<ClassElement> classesSeen = <ClassElement>[this];
       while (nearestNonMixinClass.isMixinApplication) {
         if (classesSeen.contains(nearestNonMixinClass)) {
           // Loop in the class hierarchy (which is reported elsewhere).  Don't
           // confuse the user with further errors.
           return false;
         }
         classesSeen.add(nearestNonMixinClass);
         if (nearestNonMixinClass.supertype == null) {
           // Should never happen, since Object is the only class that has no
           // supertype, and it is not a mixin application.
@@ skipping 4815 matching lines @@
 
   @override
   void visitElement(Element element) {
     int offset = element.nameOffset;
     if (offset != -1) {
       map[offset] = element;
     }
     super.visitElement(element);
   }
 }