Improve performance of rest parameter.

Rather than initing an empty array and filling, create an array of the correct size up-front. Minor gain on chromium, but considerably (~5x) faster in spidermonkey/firefox.

test/fixtures/transformation/es6-rest-parameters/arrow-functions/expected.js

@@ -1,9 +1,9 @@
 "use strict";
 
 var concat = function () {
-  var arrs = [];
-
-  for (var _key = 0; _key < arguments.length; _key++) {
+  for (var _len = arguments.length,
+      arrs = Array(_len),
+      _key = 0; _key < _len; _key++) {
     arrs[_key] = arguments[_key];
   }
 };

test/fixtures/transformation/es6-rest-parameters/multiple/expected.js

@@ -1,17 +1,17 @@
 "use strict";
 
 var t = function (f) {
-  var items = [];
-
-  for (var _key = 1; _key < arguments.length; _key++) {
+  for (var _len = arguments.length,
+      items = Array(_len > 1 ? _len - 1 : 0),
+      _key = 1; _key < _len; _key++) {
     items[_key - 1] = arguments[_key];
   }
 };
 
 function t(f) {
-  var items = [];
-
-  for (var _key2 = 1; _key2 < arguments.length; _key2++) {
+  for (var _len2 = arguments.length,
+      items = Array(_len2 > 1 ? _len2 - 1 : 0),
+      _key2 = 1; _key2 < _len2; _key2++) {
     items[_key2 - 1] = arguments[_key2];
   }
 }

test/fixtures/transformation/es6-rest-parameters/single/expected.js

@@ -1,17 +1,17 @@
 "use strict";
 
 var t = function () {
-  var items = [];
-
-  for (var _key = 0; _key < arguments.length; _key++) {
+  for (var _len = arguments.length,
+      items = Array(_len),
+      _key = 0; _key < _len; _key++) {
     items[_key] = arguments[_key];
   }
 };
 
 function t() {
-  var items = [];
-
-  for (var _key2 = 0; _key2 < arguments.length; _key2++) {
+  for (var _len2 = arguments.length,
+      items = Array(_len2),
+      _key2 = 0; _key2 < _len2; _key2++) {
     items[_key2] = arguments[_key2];
   }
 }