Fixed implementation of ArgResults.options to really use Iterable<String> instead of Iterable<dynam…

lib/src/arg_results.dart

Index: lib/src/arg_results.dart
diff --git a/lib/src/arg_results.dart b/lib/src/arg_results.dart
index de9cca7010f65f4bedfebf6da9180016beb7f1b9..15fb5bbc3c4f288e6df3288feb9387b232678447 100644
--- a/lib/src/arg_results.dart
+++ b/lib/src/arg_results.dart
@@ -70,7 +70,7 @@ class ArgResults {
   /// This includes the options whose values were parsed or that have defaults.
   /// Options that weren't present and have no default will be omitted.
   Iterable<String> get options {
-    var result = new Set.from(_parsed.keys);
+    var result = new Set<String>.from(_parsed.keys);
 
     // Include the options that have defaults.
     _parser.options.forEach((name, option) {