use slice helper on arguments instead of toArray

lib/6to5/file.js

@@ -73,6 +73,8 @@ File.normaliseOptions = function (opts) {
 File.prototype.toArray = function (node) {
   if (t.isArrayExpression(node)) {
     return node;
+  } else if (t.isIdentifier(node) && node.name === "arguments") {
+    return t.callExpression(t.memberExpression(this.addDeclaration("slice"), t.identifier("call")), [node]);
   } else {
     return t.callExpression(this.addDeclaration("to-array"), [node]);
   }