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

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

Index: pkg/analyzer/lib/dart/element/element.dart
diff --git a/pkg/analyzer/lib/dart/element/element.dart b/pkg/analyzer/lib/dart/element/element.dart
index 192485a63f79c71da3f1e1072066e774d63613db..7cb29398897e2a5679fe850b3e51399f34ca0be6 100644
--- a/pkg/analyzer/lib/dart/element/element.dart
+++ b/pkg/analyzer/lib/dart/element/element.dart
@@ -867,6 +867,13 @@ abstract class ElementAnnotation
    * value of this annotation could not be computed because of errors.
    */
   DartObject computeConstantValue();
+
+  /**
+   * Return a textual description of this annotation in a form approximating
+   * valid source. The returned string will not be valid source primarily in the
+   * case where the annotation itself is not well-formed.
+   */
+  String toSource();
 }
 
 /**