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

src/preparser.cc

Index: src/preparser.cc
diff --git a/src/preparser.cc b/src/preparser.cc
index 950f7b50ec35ca6a255788dac4b1aecccdef9086..d9f117ca8b0438086a22895437fe55e24459f766 100644
--- a/src/preparser.cc
+++ b/src/preparser.cc
@@ -983,6 +983,7 @@ PreParser::Expression PreParser::ParseFunctionLiteral(
   // Parse function body.
   bool outer_is_script_scope = scope_->is_script_scope();
   Scope* function_scope = NewScope(scope_, FUNCTION_SCOPE);
+  Scope* parameter_scope = NewScope(function_scope, PARAMETER_SCOPE);
   PreParserFactory factory(NULL);
   FunctionState function_state(&function_state_, &scope_, function_scope, kind,
                                &factory);
@@ -995,8 +996,12 @@ PreParser::Expression PreParser::ParseFunctionLiteral(
   int num_parameters;
   {
     DuplicateFinder duplicate_finder(scanner()->unicode_cache());
-    num_parameters = ParseFormalParameterList(&duplicate_finder, &error_locs,
-                                              &is_rest, CHECK_OK);
+    PreParserExpressionList initializers = NewExpressionList(0, zone());
+    bool has_initializers = false;
+
+    num_parameters = ParseFormalParameterList(
+        &duplicate_finder, parameter_scope, &error_locs, initializers,
+        &has_initializers, &is_rest, CHECK_OK);
   }
   Expect(Token::RPAREN, CHECK_OK);
   int formals_end_position = scanner()->location().end_pos;