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

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

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library analyzer.test.src.task.incremental_element_builder_test;
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/src/dart/ast/utilities.dart';
 import 'package:analyzer/src/dart/element/element.dart';
@@ skipping 667 matching lines @@
     expect(elementB.name, 'bbb');
     // verify delta
     expect(helper.delta.addedConstructors, isEmpty);
     expect(helper.delta.removedConstructors, isEmpty);
     expect(helper.delta.addedAccessors, isEmpty);
     expect(helper.delta.removedAccessors, isEmpty);
     expect(helper.delta.addedMethods, unorderedEquals([newElementA]));
     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'''
 class A {}
 class B {}
 ''',
         r'''
 class A {}
 class B extends A {}
 ''');
@@ skipping 1177 matching lines @@
     expect(oldClass, isNotNull);
     element = oldClass.element;
     oldVersion = element.version;
     oldMembers = oldClass.members.toList();
   }
 
   ClassDeclaration _findClassNode(CompilationUnit unit, String name) =>
       unit.declarations.singleWhere((unitMember) =>
           unitMember is ClassDeclaration && unitMember.name.name == name);
 }