migrate raw string to new syntax in pkg

pkg/args/lib/args.dart

Index: pkg/args/lib/args.dart
===================================================================
--- pkg/args/lib/args.dart	(revision 12745)
+++ pkg/args/lib/args.dart	(working copy)
@@ -195,9 +195,9 @@
  * options and flags from them.
  */
 class ArgParser {
-  static const _SOLO_OPT = const RegExp(@'^-([a-zA-Z0-9])$');
-  static const _ABBR_OPT = const RegExp(@'^-([a-zA-Z0-9]+)(.*)$');
-  static const _LONG_OPT = const RegExp(@'^--([a-zA-Z\-_0-9]+)(=(.*))?$');
+  static const _SOLO_OPT = const RegExp(r'^-([a-zA-Z0-9])$');
+  static const _ABBR_OPT = const RegExp(r'^-([a-zA-Z0-9]+)(.*)$');
+  static const _LONG_OPT = const RegExp(r'^--([a-zA-Z\-_0-9]+)(=(.*))?$');
 
   final Map<String, _Option> _options;