[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..7c26755295bc2e311bb9e48eb3d5e3c1af0ba1a4 100644
--- a/src/parser.h
+++ b/src/parser.h
@@ -746,9 +746,10 @@ class ParserTraits {
                             FunctionKind kind = kNormalFunction);
 
   bool DeclareFormalParameter(Scope* scope, const AstRawString* name,
-                              bool is_rest) {
+                              bool is_rest, int pos) {
     bool is_duplicate = false;
-    Variable* var = scope->DeclareParameter(name, VAR, is_rest, &is_duplicate);
+    Variable* var =
+        scope->DeclareParameter(name, VAR, pos, is_rest, &is_duplicate);
     if (is_sloppy(scope->language_mode())) {
       // TODO(sigurds) Mark every parameter as maybe assigned. This is a
       // conservative approximation necessary to account for parameters
@@ -977,7 +978,8 @@ 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(
+      Scope* scope, bool has_initializers, ZoneList<Expression*>* initializers);
   FunctionLiteral* ParseFunctionLiteral(
       const AstRawString* name, Scanner::Location function_name_location,
       bool name_is_strict_reserved, FunctionKind kind,
@@ -1009,7 +1011,18 @@ class Parser : public ParserBase<ParserTraits> {
 
   // Parser support
   VariableProxy* NewUnresolved(const AstRawString* name, VariableMode mode);
-  Variable* Declare(Declaration* declaration, bool resolve, bool* ok);
+  Variable* Declare(Declaration* declaration, bool resolve,
+                    bool allow_redeclaration, bool* ok);
+  Variable* Declare(Declaration* declaration, bool resolve,
+                    bool allow_redeclaration) {
+    bool ok = true;
+    Variable* var = Declare(declaration, resolve, allow_redeclaration, &ok);
+    DCHECK(ok);
+    return var;
+  }
+  Variable* Declare(Declaration* declaration, bool resolve, bool* ok) {
+    return Declare(declaration, resolve, false, ok);
+  }
 
   bool TargetStackContainsLabel(const AstRawString* label);
   BreakableStatement* LookupBreakTarget(const AstRawString* label, bool* ok);