fix rest parameter keys when on functions with params

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

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

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

@@ -12,6 +12,14 @@ exports.Function = function (node, parent, file) {
   var argsId = t.identifier("arguments");
   argsId._ignoreAliasFunctions = true;
 
+  var start = t.literal(node.params.length);
+  var key = file.generateUidIdentifier("key");
+
+  var arrKey = key;
+  if (node.params.length) {
+    arrKey = t.binaryExpression("-", arrKey, start);
+  }
+
   node.body.body.unshift(
     t.variableDeclaration("var", [
       t.variableDeclarator(rest, t.arrayExpression([]))
@@ -19,9 +27,10 @@ exports.Function = function (node, parent, file) {
 
     util.template("rest", {
       ARGUMENTS: argsId,
-      START: t.literal(node.params.length),
+      ARRAY_KEY: arrKey,
+      START: start,
       ARRAY: rest,
-      KEY: file.generateUidIdentifier("key")
+      KEY: key
     })
   );
 };