[es6] implement default parameters via desugaring

src/preparser.cc

Index: src/preparser.cc
diff --git a/src/preparser.cc b/src/preparser.cc
index e3c421ea140b8c3da40156c548880df358a2d4c1..659e40a0f12d7b2213b61fb28b2410da9c119768 100644
--- a/src/preparser.cc
+++ b/src/preparser.cc
@@ -1031,10 +1031,13 @@ PreParser::Expression PreParser::ParseFunctionLiteral(
   int start_position = scanner()->location().beg_pos;
   function_scope->set_start_position(start_position);
   int num_parameters;
+  bool has_initializers = false;
   {
     DuplicateFinder duplicate_finder(scanner()->unicode_cache());
-    num_parameters = ParseFormalParameterList(&duplicate_finder, &error_locs,
-                                              &is_rest, CHECK_OK);
+    PreParserExpressionList initializers = NewExpressionList(0, zone());
+    num_parameters =
+        ParseFormalParameterList(&duplicate_finder, &error_locs, initializers,
+                                 &has_initializers, &is_rest, CHECK_OK);
   }
   Expect(Token::RPAREN, CHECK_OK);
   int formals_end_position = scanner()->location().end_pos;