Fix warning in test of js_ast package.

pkg/js_ast/test/printer_callback_test.dart

 // Copyright (c) 2015, 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.
 
 // Note: This test relies on LF line endings in the source file.
 
 // Test that JS printer callbacks occur when expected.
 
 library js_ast.printer.callback_test;
 
@@ skipping 153 matching lines @@
     TestMode.INPUT: "a.#nameTemplate",
     TestMode.NONE: "a.nameValue",
     TestMode.ENTER: "@0@1a.@2nameValue",
     TestMode.DELIMITER: "a.nameValue",
     TestMode.EXIT: "a@1.nameValue@2@0",
   }, const {'nameTemplate': 'nameValue'}),
 ];
 
 class FixedName extends Name {
   final String name;
+  String get key => name;
 
   FixedName(this.name);
 
   @override
   int compareTo(other) => 0;
 }
 
 void check(TestCase testCase) {
   Map<TestMode, String> map = testCase.data;
   String code = map[TestMode.INPUT];
@@ skipping 62 matching lines @@
     if (offset < text.length) {
       sb.write(text.substring(offset));
     }
     return sb.toString();
   }
 }
 
 void main() {
   DATA.forEach(check);
 }