Handle unexpected tokens in modifier lists.

tests/language/regress_23051_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 issue 23051.
+
+main() {
+  new A();                                 /// 01: compile-time error
+}
+
+class A {                                  /// 01: continued
+  // Note the trailing ' in the next line. /// 01: continued
+  get foo => bar();'                       /// 01: continued
+                                           /// 01: continued
+  String bar(                              /// 01: continued