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

pkg/analyzer/lib/src/dart/element/element.dart

Index: pkg/analyzer/lib/src/dart/element/element.dart
diff --git a/pkg/analyzer/lib/src/dart/element/element.dart b/pkg/analyzer/lib/src/dart/element/element.dart
index 2175e9e3078502cbd5d5ee324ae8954da0cd61bc..bcac6b1e3051481fe0cbd7113d66ab579533ba7e 100644
--- a/pkg/analyzer/lib/src/dart/element/element.dart
+++ b/pkg/analyzer/lib/src/dart/element/element.dart
@@ -208,7 +208,7 @@ class ClassElementImpl extends ElementImpl implements ClassElement {
       // 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