Do not fail when an enum constant is annotated (issue 26413)

pkg/analyzer/lib/src/dart/constant/utilities.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.constant.utilities;
 
 import 'dart:collection';
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/visitor.dart';
@@ skipping 154 matching lines @@
    */
   bool treatFinalInstanceVarAsConst = false;
 
   ConstantFinder(this.context, this.source, this.librarySource);
 
   @override
   Object visitAnnotation(Annotation node) {
     super.visitAnnotation(node);
     ElementAnnotation elementAnnotation = node.elementAnnotation;
     if (elementAnnotation == null) {
-      // Analyzer ignores annotations on "part of" directives.
-      assert(node.parent is PartOfDirective);
+      // Analyzer ignores annotations on "part of" directives and on enum
+      // constant declarations.
+      assert(node.parent is PartOfDirective ||
+          node.parent is EnumConstantDeclaration);
     } else {
       constantsToCompute.add(elementAnnotation);
     }
     return null;
   }
 
   @override
   Object visitClassDeclaration(ClassDeclaration node) {
     bool prevTreatFinalInstanceVarAsConst = treatFinalInstanceVarAsConst;
     if (node.element.constructors.any((ConstructorElement e) => e.isConst)) {
@@ skipping 116 matching lines @@
   @override
   Object visitSuperConstructorInvocation(SuperConstructorInvocation node) {
     super.visitSuperConstructorInvocation(node);
     ConstructorElement constructor = getConstructorImpl(node.staticElement);
     if (constructor != null) {
       _callback(constructor);
     }
     return null;
   }
 }