Allow trailing commas in function parameter lists

src/parsing/parser.cc

 // Copyright 2012 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.
 
 #include "src/parsing/parser.h"
 
 #include "src/api.h"
 #include "src/ast/ast.h"
 #include "src/ast/ast-expression-rewriter.h"
 #include "src/ast/ast-expression-visitor.h"
@@ skipping 798 matching lines @@
   set_allow_harmony_do_expressions(FLAG_harmony_do_expressions);
   set_allow_harmony_for_in(FLAG_harmony_for_in);
   set_allow_harmony_function_name(FLAG_harmony_function_name);
   set_allow_harmony_function_sent(FLAG_harmony_function_sent);
   set_allow_harmony_restrictive_declarations(
       FLAG_harmony_restrictive_declarations);
   set_allow_harmony_exponentiation_operator(
       FLAG_harmony_exponentiation_operator);
   set_allow_harmony_async_await(FLAG_harmony_async_await);
   set_allow_harmony_restrictive_generators(FLAG_harmony_restrictive_generators);
+  set_allow_harmony_trailing_commas_in_parameters(
+      FLAG_harmony_trailing_commas_in_parameters);

[caitp (gmail), 2016/06/22 18:56:20]

You need to do something similar in Parser::ParseLazyFunctionBodyWithPreParser()
so the preparser has the same flags

   for (int feature = 0; feature < v8::Isolate::kUseCounterFeatureCount;
        ++feature) {
     use_counts_[feature] = 0;
   }
   if (info->ast_value_factory() == NULL) {
     // info takes ownership of AstValueFactory.
     info->set_ast_value_factory(new AstValueFactory(zone(), info->hash_seed()));
     info->set_ast_value_factory_owned();
     ast_value_factory_ = info->ast_value_factory();
   }
@@ skipping 6150 matching lines @@
                         try_block, target);
     final_loop = target;
   }
 
   return final_loop;
 }
 
 
 }  // namespace internal
 }  // namespace v8