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.

lib/6to5/transformation/templates/rest.js

@@ -1,3 +1,3 @@
-for (var KEY = START; KEY < ARGUMENTS.length; KEY++) {
+for (var LEN = ARGUMENTS.length, ARRAY = Array(ARRAY_LEN), KEY = START; KEY < LEN; KEY++) {
   ARRAY[ARRAY_KEY] = ARGUMENTS[KEY];
 }

lib/6to5/transformation/transformers/es6-rest-parameters.js

@@ -16,26 +16,34 @@ exports.Function = function (node, parent, file) {
 
   var start = t.literal(node.params.length);
   var key = file.generateUidIdentifier("key");
+  var len = file.generateUidIdentifier("len");
 
   var arrKey = key;
   if (node.params.length) {
     // this method has additional params, so we need to subtract
     // the index of the current argument position from the
     // position in the array that we want to populate
-    arrKey = t.binaryExpression("-", arrKey, start);
+    arrKey = t.binaryExpression("-", key, start);
+  }
+
+  var arrLen = len;
+  if (node.params.length) {
+    arrLen = t.conditionalExpression(
+      t.binaryExpression(">", len, start),
+      t.binaryExpression("-", len, start),
+      t.literal(0)
+    );
   }
 
   node.body.body.unshift(
-    t.variableDeclaration("var", [
-      t.variableDeclarator(rest, t.arrayExpression([]))
-    ]),
-
     util.template("rest", {
       ARGUMENTS: argsId,
       ARRAY_KEY: arrKey,
+      ARRAY_LEN: arrLen,
       START: start,
       ARRAY: rest,
-      KEY: key
+      KEY: key,
+      LEN: len,
     })
   );
 };