dart2js: allow trailing commas in parameter and argument lists

tests/compiler/dart2js_extra/invalid_annotation2_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.
 
 // Regression test for http://dartbug.com/23893/
 //
 // Because annotations are parsed lazily, dart2js used to crash when an
 // annotation had a syntax error.
 // This checks the same behavior as invalid_annotation_test.dart, except that we
 // use mirrors to trigger the error in the vm. This also triggers the error in
 // dart2js differently.
 
 @MirrorsUsed(targets: const [A])
 import 'dart:mirrors';
 
 @Deprecated("m"
-,                                /// 01: compile-time error
+,,                                /// 01: compile-time error
 )
 class A {
 }
 
 main() {
   reflectClass(A).metadata;
 }