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

pkg/analyzer/lib/src/task/incremental_element_builder.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.src.task.incremental_element_builder;
 
 import 'dart:collection';
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/token.dart';
@@ skipping 137 matching lines @@
       unitElementHolder.addAccessor(element);
     } else if (element is TopLevelVariableElement) {
       unitElementHolder.addTopLevelVariable(element);
     }
   }
 
   ClassElementDelta _processClassMembers(
       ClassDeclaration oldClass, ClassDeclaration newClass) {
     // If the class hierarchy or type parameters are changed,
     // then the class changed too much - don't compute the delta.
-    if (TokenUtils.getFullCode(newClass.typeParameters) !=
+    if (newClass.abstractKeyword != null && oldClass.abstractKeyword == null ||
+        newClass.abstractKeyword == null && oldClass.abstractKeyword != null ||
+        TokenUtils.getFullCode(newClass.typeParameters) !=
             TokenUtils.getFullCode(oldClass.typeParameters) ||
         TokenUtils.getFullCode(newClass.extendsClause) !=
             TokenUtils.getFullCode(oldClass.extendsClause) ||
         TokenUtils.getFullCode(newClass.withClause) !=
             TokenUtils.getFullCode(oldClass.withClause) ||
         TokenUtils.getFullCode(newClass.implementsClause) !=
             TokenUtils.getFullCode(oldClass.implementsClause)) {
       return null;
     }
     // Build the old class members map.
@@ skipping 526 matching lines @@
       }
     }
     super.visitElement(element);
   }
 
   static bool _isVariableInitializer(Element element) {
     return element is FunctionElement &&
         element.enclosingElement is VariableElement;
   }
 }