Revert "Improve performance of rest parameter."

This reverts commit e5463cc2a9.

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

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

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

@@ -14,31 +14,23 @@ 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) {
-    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)
-    );
+    arrKey = t.binaryExpression("-", arrKey, start);
   }
 
   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,
-      LEN: len,
+      KEY: key
     })
   );
 };