Adding/removing 'abstract' keyword is not a class delta change.

pkg/analyzer/test/src/task/incremental_element_builder_test.dart

Index: pkg/analyzer/test/src/task/incremental_element_builder_test.dart
diff --git a/pkg/analyzer/test/src/task/incremental_element_builder_test.dart b/pkg/analyzer/test/src/task/incremental_element_builder_test.dart
index 77aff5b9fc91b332cfe618eb75fff2a638ef59df..a605a69cd4f6ce4acebb0d172170c66fb583b2e4 100644
--- a/pkg/analyzer/test/src/task/incremental_element_builder_test.dart
+++ b/pkg/analyzer/test/src/task/incremental_element_builder_test.dart
@@ -685,6 +685,26 @@ class A {
     expect(helper.delta.removedMethods, unorderedEquals([oldElementA]));
   }
 
+  test_classDelta_null_abstractKeyword_add() {
+    _verifyNoClassDeltaForTheLast(
+        r'''
+class A {}
+''',
+        r'''
+abstract class A {}
+''');
+  }
+
+  test_classDelta_null_abstractKeyword_remove() {
+    _verifyNoClassDeltaForTheLast(
+        r'''
+abstract class A {}
+''',
+        r'''
+class A {}
+''');
+  }
+
   test_classDelta_null_extendsClause_add() {
     _verifyNoClassDeltaForTheLast(
         r'''