Add toSource() to ElementAnnotation, ElementAnnotationImpl (issue 28631)

pkg/analyzer/lib/src/dart/element/element.dart

Index: pkg/analyzer/lib/src/dart/element/element.dart
diff --git a/pkg/analyzer/lib/src/dart/element/element.dart b/pkg/analyzer/lib/src/dart/element/element.dart
index c669ae12c086b4fa0e813ac22a833ebe6001aeff..af93dd3e3a17f4eb7de1c3e340f5a7c6a6a6bda8 100644
--- a/pkg/analyzer/lib/src/dart/element/element.dart
+++ b/pkg/analyzer/lib/src/dart/element/element.dart
@@ -2663,6 +2663,9 @@ class ElementAnnotationImpl implements ElementAnnotation {
   }
 
   @override
+  String toSource() => annotationAst.toSource();
+
+  @override
   String toString() => '@$element';
 }