Fix error recovery for extracting native name.

pkg/front_end/lib/src/fasta/parser/dart_vm_native.dart

 // Copyright (c) 2017, 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.
 
 /// Implements support for Dart VM native method bodies of this form:
 ///
 ///     native STRING
 ///
 /// This support is kept separate from parser.dart as this isn't specified in
 /// the Dart Language Specification, also we hope to remove this syntax long
@@ skipping 27 matching lines @@
 ///
 /// This will result in [identifiers] being
 ///
 ///     [";", '"List_getLength"', "native", "length", "get"]
 ///
 /// We need to remove '"List_getLength"' and "native" from that list.
 ///
 /// This method designed to be called from [Listener.handleMemberName].
 Link<Token> removeNativeClause(Link<Token> identifiers) {
   Link<Token> result = identifiers.tail;
+  if (result.isEmpty) return identifiers;
   if (result.head.kind != STRING_TOKEN) return identifiers;
   result = result.tail;
   if (result.isEmpty) return identifiers;
   if (optional('native', result.head)) {
     return result.tail.prepend(identifiers.head);
   }
   return identifiers;
 }