Infer only ClassElementImpl in InstanceMemberInferrer, tweaks for link.

pkg/analyzer/lib/src/task/strong_mode.dart

Index: pkg/analyzer/lib/src/task/strong_mode.dart
diff --git a/pkg/analyzer/lib/src/task/strong_mode.dart b/pkg/analyzer/lib/src/task/strong_mode.dart
index efe5637647540350b5331c6a4a0a269c3ea0dda2..420fd61c972aa62d41a511067e458889fac32c52 100644
--- a/pkg/analyzer/lib/src/task/strong_mode.dart
+++ b/pkg/analyzer/lib/src/task/strong_mode.dart
@@ -73,8 +73,8 @@ class InstanceMemberInferrer {
    * The classes that have been visited while attempting to infer the types of
    * instance members of some base class.
    */
-  HashSet<AbstractClassElementImpl> elementsBeingInferred =
-      new HashSet<AbstractClassElementImpl>();
+  HashSet<ClassElementImpl> elementsBeingInferred =
+      new HashSet<ClassElementImpl>();
 
   /**
    * Initialize a newly create inferrer.
@@ -202,7 +202,7 @@ class InstanceMemberInferrer {
    * [classElement].
    */
   void _inferClass(ClassElement classElement) {
-    if (classElement is AbstractClassElementImpl) {
+    if (classElement is ClassElementImpl) {
       if (classElement.hasBeenInferred) {
         return;
       }