don't crash when given no arguments

bin/dartdevc.dart

Index: bin/dartdevc.dart
diff --git a/bin/dartdevc.dart b/bin/dartdevc.dart
index a4db40cb751a3052e6d118726c389b7a6571f7f1..0ee480ac83faca90afbbc6f5ab5cc0e827c6ac7f 100755
--- a/bin/dartdevc.dart
+++ b/bin/dartdevc.dart
@@ -57,7 +57,7 @@ main(List<String> args) async {
 /// Bazel actions that support workers must provide all their per-WorkRequest
 /// arguments in a file like this instead of as normal args.
 List<String> _preprocessArgs(List<String> args) {
-  if (args.last.startsWith('@')) {
+  if (args.isNotEmpty && args.last.startsWith('@')) {
     return new List.from(args)
       ..addAll(new File(args.last.substring(1)).readAsLinesSync());
   } else {