Fix errors introduced by recent strong-mode changes

pkg/analysis_server/test/integration/search/get_type_hierarchy_test.dart

Index: pkg/analysis_server/test/integration/search/get_type_hierarchy_test.dart
diff --git a/pkg/analysis_server/test/integration/search/get_type_hierarchy_test.dart b/pkg/analysis_server/test/integration/search/get_type_hierarchy_test.dart
index a4ab04e43d78eeefa0af6691a65a6e0a599948dd..5450e60b15a0b449f248ba5888a821196d7b57a4 100644
--- a/pkg/analysis_server/test/integration/search/get_type_hierarchy_test.dart
+++ b/pkg/analysis_server/test/integration/search/get_type_hierarchy_test.dart
@@ -103,6 +103,7 @@ class Derived extends Pivot {}
               equals(text.indexOf('class $name') + 'class '.length));
         }
       }
+
       checkElement('Object');
       checkElement('Base');
       checkElement('Pivot');
@@ -270,7 +271,8 @@ class Pivot /* target */ extends Base2 {}
       if (result.hierarchyItems == null) {
         return null;
       } else {
-        return new HierarchyResults(result.hierarchyItems);
+        return new HierarchyResults(
+            result.hierarchyItems as List<TypeHierarchyItem>);
       }
     });
   }