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

pkg/analyzer/lib/src/dart/element/element.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.src.dart.element.element;
 
 import 'dart:collection';
 import 'dart:math' show min;
 
 import 'package:analyzer/dart/ast/ast.dart';
@@ skipping 2645 matching lines @@
 
   @override
   DartObject computeConstantValue() {
     if (evaluationResult == null) {
       context?.computeResult(this, CONSTANT_VALUE);
     }
     return constantValue;
   }
 
   @override
+  String toSource() => annotationAst.toSource();
+
+  @override
   String toString() => '@$element';
 }
 
 /**
  * A base class for concrete implementations of an [Element].
  */
 abstract class ElementImpl implements Element {
   /**
    * An Unicode right arrow.
    */
@@ skipping 5970 matching lines @@
 
   @override
   void visitElement(Element element) {
     int offset = element.nameOffset;
     if (offset != -1) {
       map[offset] = element;
     }
     super.visitElement(element);
   }
 }