Make arg parser return growable list of rest arguments.

pkg/args/lib/src/parser.dart

Index: pkg/args/lib/src/parser.dart
diff --git a/pkg/args/lib/src/parser.dart b/pkg/args/lib/src/parser.dart
index d35f9ea4aff1bb67a1c20034b17574168d0c6502..5c372ab0e8065d958cfd3da90d96564433426272 100644
--- a/pkg/args/lib/src/parser.dart
+++ b/pkg/args/lib/src/parser.dart
@@ -95,7 +95,7 @@ class Parser {
     });
 
     // Add in the leftover arguments we didn't parse to the innermost command.
-    var rest = args.toList();
+    var rest = args.toList(growable: true);
     args.clear();
     return new ArgResults(results, commandName, commandResults, rest);
   }