Allow trailing commas in function parameter lists

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
+
+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);