Some more improvement to the Dart coding style.

luhn.dart

@@ -84,19 +84,17 @@ class Luhn {
   }
 
   void tapStdin() {
-    var s = "";
     var args = new Options().arguments;
     var nRepeats = args.length > 0 ? Math.parseInt(args[0]) : 1;
     if (nRepeats > 1) {
       var lines = [];
       readRawLines((s) => lines.add(s));
       for (var i = 0; i < nRepeats; i++) {
-        for (s in lines) {
+        for (var s in lines) {
           print(new String.fromCharCodes(mask(s)));
         }
       }
     } else {
-      var a;
       readRawLines((a) => print(new String.fromCharCodes(mask(a))));
     }
     exit(0);