Another fix incremental constants - annotations should use the old unit element.

pkg/analyzer/test/src/context/context_test.dart

 // Copyright (c) 2014, 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.context.context_test;
 
 import 'dart:async';
 import 'dart:collection';
 
 import 'package:analyzer/dart/ast/ast.dart';
@@ skipping 3294 matching lines @@
         r'''
 int _A = 2;
 int B = _A + 1;
 ''');
     _assertValidForChangedLibrary(a);
     _assertInvalid(a, LIBRARY_ERRORS_READY);
     _assertValidForDependentLibrary(b);
     _assertInvalid(b, LIBRARY_ERRORS_READY);
   }
 
+  void test_sequence_add_annotation() {
+    Source a = addSource(
+        '/a.dart',
+        r'''
+const myAnnotation = const Object();
+class A {}
+''');
+    _performPendingAnalysisTasks();
+    // Add a new annotation.
+    context.setContents(
+        a,
+        r'''
+const myAnnotation = const Object();
+@myAnnotation
+class A {}
+''');
+    _assertValidForChangedLibrary(a);
+    _assertInvalid(a, LIBRARY_ERRORS_READY);
+    // Analysis is done successfully.
+    _performPendingAnalysisTasks();
+    _assertValid(a, LIBRARY_ERRORS_READY);
+    _assertValid(a, READY_RESOLVED_UNIT);
+  }
+
   void test_sequence_class_give_take() {
     Source a = addSource(
         '/a.dart',
         r'''
 class A {}
 class B {}
 class C {}
 ''');
     Source b = addSource(
         '/b.dart',
@@ skipping 915 matching lines @@
    * Initialize the visitor.
    */
   _ElementGatherer();
 
   @override
   void visitElement(Element element) {
     elements[element] = element;
     super.visitElement(element);
   }
 }