Enable --harmony-trailing-commas

test/mjsunit/harmony/trailing-commas-length.js

 // Copyright 2016 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-trailing-commas

[adamk, 2017/01/30 22:41:03]

You can leave this flag here until it's actually removed from flag-definitions.h

-
 function f1(a,) {}
 function f2(a,b,) {}
 function f3(a,b,c,) {}
 assertEquals(1, f1.length);
 assertEquals(2, f2.length);
 assertEquals(3, f3.length);
 
 function* g1(a,) {}
 function* g2(a,b,) {}
 function* g3(a,b,c,) {}
 assertEquals(1, g1.length);
 assertEquals(2, g2.length);
 assertEquals(3, g3.length);
 
 assertEquals(1, (function(a,) {}).length);
 assertEquals(2, (function(a,b,) {}).length);
 assertEquals(3, (function(a,b,c,) {}).length);
 
 assertEquals(1, (function*(a,) {}).length);
 assertEquals(2, (function*(a,b,) {}).length);
 assertEquals(3, (function*(a,b,c,) {}).length);
 
 assertEquals(1, ((a,) => {}).length);
 assertEquals(2, ((a,b,) => {}).length);
 assertEquals(3, ((a,b,c,) => {}).length);