Stop looking for overrides once the pivot element reached.

pkg/analysis_server/lib/src/search/type_hierarchy.dart

Index: pkg/analysis_server/lib/src/search/type_hierarchy.dart
diff --git a/pkg/analysis_server/lib/src/search/type_hierarchy.dart b/pkg/analysis_server/lib/src/search/type_hierarchy.dart
index a4b5837ec18685f3184da445a93187e5e66dfbf2..695693cf7d9304693b1b7bfb92542469c89cac7e 100644
--- a/pkg/analysis_server/lib/src/search/type_hierarchy.dart
+++ b/pkg/analysis_server/lib/src/search/type_hierarchy.dart
@@ -131,9 +131,8 @@ class TypeHierarchyComputer {
       ExecutableElement memberElement = _findMemberElement(classElement);
       item = new TypeHierarchyItem(convertElement(classElement),
           displayName: displayName,
-          memberElement: memberElement != null
-              ? convertElement(memberElement)
-              : null);
+          memberElement:
+              memberElement != null ? convertElement(memberElement) : null);
       _elementItemMap[classElement] = item;
       itemId = _items.length;
       _items.add(item);
@@ -193,6 +192,9 @@ class TypeHierarchyComputer {
           result = mixinElement.lookUpSetter(_pivotName, _pivotLibrary);
         }
       }
+      if (result == _pivotElement) {
+        return null;
+      }
       if (result != null) {
         return result;
       }