Use ElementAnnotation as the ConstantEvaluationTarget for annotations.

pkg/analyzer/lib/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.dart.element.element;
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/element/type.dart';
 import 'package:analyzer/src/generated/constant.dart' show DartObject;
 import 'package:analyzer/src/generated/engine.dart' show AnalysisContext;
@@ skipping 726 matching lines @@
    * is no guarantee of the order in which the children will be visited.
    */
   void visitChildren(ElementVisitor visitor);
 }
 
 /**
  * A single annotation associated with an element.
  *
  * Clients may not extend, implement or mix-in this class.
  */
-abstract class ElementAnnotation {
+abstract class ElementAnnotation implements ConstantEvaluationTarget {
   /**
    * An empty list of annotations.
    */
   static const List<ElementAnnotation> EMPTY_LIST = const <ElementAnnotation>[];
 
   /**
    * Return a representation of the value of this annotation.
    *
    * Return `null` if the value of this annotation could not be computed because
    * of errors.
@@ skipping 1143 matching lines @@
    */
   bool get isStatic;
 
   /**
    * Return the declared type of this variable, or `null` if the variable did
    * not have a declared type (such as if it was declared using the keyword
    * 'var').
    */
   DartType get type;
 }