[es6] Correct length for functions with default parameters

test/mjsunit/harmony/default-parameters.js

 // Copyright 2015 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // Flags: --harmony-default-parameters --harmony-arrow-functions
 // Flags: --harmony-rest-parameters --harmony-destructuring
 
 
 (function TestDefaults() {
   function f1(x = 1) { return x }
@@ skipping 400 matching lines @@
   function f1(x = 900) { arguments[0] = 1; return x }
   assertEquals(9, f1(9));
   assertEquals(900, f1());
   function f2(x = 1001) { x = 2; return arguments[0] }
   assertEquals(10, f2(10));
   assertEquals(undefined, f2());
 }());
 
 
 (function TestFunctionLength() {
-  // TODO(rossberg): Fix arity.
-  // assertEquals(0, (function(x = 1) {}).length);
-  // assertEquals(0, (function(x = 1, ...a) {}).length);
-  // assertEquals(1, (function(x, y = 1) {}).length);
-  // assertEquals(1, (function(x, y = 1, ...a) {}).length);
-  // assertEquals(2, (function(x, y, z = 1) {}).length);
-  // assertEquals(2, (function(x, y, z = 1, ...a) {}).length);
-  // assertEquals(1, (function(x, y = 1, z) {}).length);
-  // assertEquals(1, (function(x, y = 1, z, ...a) {}).length);
-  // assertEquals(1, (function(x, y = 1, z, v = 2) {}).length);
-  // assertEquals(1, (function(x, y = 1, z, v = 2, ...a) {}).length);
+   assertEquals(0, (function(x = 1) {}).length);
+   assertEquals(0, (function(x = 1, ...a) {}).length);
+   assertEquals(1, (function(x, y = 1) {}).length);
+   assertEquals(1, (function(x, y = 1, ...a) {}).length);
+   assertEquals(2, (function(x, y, z = 1) {}).length);
+   assertEquals(2, (function(x, y, z = 1, ...a) {}).length);
+   assertEquals(1, (function(x, y = 1, z) {}).length);
+   assertEquals(1, (function(x, y = 1, z, ...a) {}).length);
+   assertEquals(1, (function(x, y = 1, z, v = 2) {}).length);
+   assertEquals(1, (function(x, y = 1, z, v = 2, ...a) {}).length);
 })();