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

src/scopes.h

Index: src/scopes.h
diff --git a/src/scopes.h b/src/scopes.h
index 5e3dc1f0654bf65c3ee738665fcb7bf2107be63b..3b42ed11b275d4a491f37987ef201e0c728ba7ad 100644
--- a/src/scopes.h
+++ b/src/scopes.h
@@ -101,7 +101,7 @@ class Scope: public ZoneObject {
   // Declarations
 
   // Lookup a variable in this scope. Returns the variable or NULL if not found.
-  Variable* LookupLocal(const AstRawString* name);
+  Variable* LookupLocal(const AstRawString* name, bool parameters_only = false);
 
   // This lookup corresponds to a lookup in the "intermediate" scope sitting
   // between this scope and the outer scope. (ECMA-262, 3rd., requires that
@@ -282,6 +282,7 @@ class Scope: public ZoneObject {
   bool is_block_scope() const { return scope_type_ == BLOCK_SCOPE; }
   bool is_with_scope() const { return scope_type_ == WITH_SCOPE; }
   bool is_arrow_scope() const { return scope_type_ == ARROW_SCOPE; }
+  bool is_parameter_scope() const { return scope_type_ == PARAMETER_SCOPE; }
   void tag_as_class_scope() {
     DCHECK(is_block_scope());
     block_scope_is_class_scope_ = true;
@@ -677,7 +678,8 @@ class Scope: public ZoneObject {
   // scope. If the code is executed because of a call to 'eval', the context
   // parameter should be set to the calling context of 'eval'.
   Variable* LookupRecursive(VariableProxy* proxy, BindingKind* binding_kind,
-                            AstNodeFactory* factory);
+                            AstNodeFactory* factory,
+                            bool parameters_only = false);
   MUST_USE_RESULT
   bool ResolveVariable(ParseInfo* info, VariableProxy* proxy,
                        AstNodeFactory* factory);