[es6] implement optional parameters via desugaring (with scoping)

src/parser.h

Index: src/parser.h
diff --git a/src/parser.h b/src/parser.h
index 58253e1bb6919309a444864f10bfff904efb837f..68db87e74259cea1d0345cd4b4e233ab83ff365b 100644
--- a/src/parser.h
+++ b/src/parser.h
@@ -977,7 +977,8 @@ class Parser : public ParserBase<ParserTraits> {
       Scope* inner_scope, bool is_const, ZoneList<const AstRawString*>* names,
       ForStatement* loop, Statement* init, Expression* cond, Statement* next,
       Statement* body, bool* ok);
-
+  ZoneList<Statement*>* DesugarInitializeParameters(
+        bool has_initializers, ZoneList<Expression*>* initializers);
   FunctionLiteral* ParseFunctionLiteral(
       const AstRawString* name, Scanner::Location function_name_location,
       bool name_is_strict_reserved, FunctionKind kind,