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

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

Index: pkg/analyzer/test/src/context/context_test.dart
diff --git a/pkg/analyzer/test/src/context/context_test.dart b/pkg/analyzer/test/src/context/context_test.dart
index ca10e4b814f9c128f4e759758030f04f743d3187..43b194c83c94ae7f0cefecf01aa5b8bdac29b515 100644
--- a/pkg/analyzer/test/src/context/context_test.dart
+++ b/pkg/analyzer/test/src/context/context_test.dart
@@ -3312,6 +3312,30 @@ int B = _A + 1;
     _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',