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

src/scopes.cc

Index: src/scopes.cc
diff --git a/src/scopes.cc b/src/scopes.cc
index 8b623f90ce908ac548bfd8ea46bdcf088121bbcf..a011dd1ca82b59680f74c14e008fdb3e322ce4b3 100644
--- a/src/scopes.cc
+++ b/src/scopes.cc
@@ -150,6 +150,7 @@ void Scope::SetDefaults(ScopeType scope_type, Scope* outer_scope,
                         Handle<ScopeInfo> scope_info,
                         FunctionKind function_kind) {
   outer_scope_ = outer_scope;
+  function_body_ = nullptr;
   scope_type_ = scope_type;
   function_kind_ = function_kind;
   block_scope_is_class_scope_ = false;
@@ -194,6 +195,12 @@ void Scope::SetDefaults(ScopeType scope_type, Scope* outer_scope,
     block_scope_is_class_scope_ = scope_info->block_scope_is_class_scope();
     function_kind_ = scope_info->function_kind();
   }
+  if (scope_type == FUNCTION_BODY_SCOPE) {
+    DCHECK_NOT_NULL(outer_scope);
+    DCHECK(outer_scope->is_function_scope());
+    DCHECK_NULL(outer_scope->function_body_);
+    outer_scope->function_body_ = this;
+  }
 }
 
 
@@ -543,7 +550,12 @@ Variable* Scope::NewTemporary(const AstRawString* name) {
 
 
 void Scope::AddDeclaration(Declaration* declaration) {
-  decls_.Add(declaration, zone());
+  Scope* target = this;
+  if (is_function_body_scope() && declaration->mode() == VAR) {
+    // Add `var` declarations to outer scope, to make allocation simpler
+    target = outer_scope();
+  }
+  target->decls_.Add(declaration, zone());
 }
 
 
@@ -819,6 +831,8 @@ static const char* Header(ScopeType scope_type) {
     case BLOCK_SCOPE: return "block";
     case WITH_SCOPE: return "with";
     case ARROW_SCOPE: return "arrow";
+    case FUNCTION_BODY_SCOPE:
+      return "function body";
   }
   UNREACHABLE();
   return NULL;
@@ -1356,7 +1370,11 @@ bool Scope::HasArgumentsParameter(Isolate* isolate) {
 
 
 void Scope::AllocateStackSlot(Variable* var) {
-  if (is_block_scope()) {
+  if (is_function_body_scope()) {
+    // Function body variables need to be allocated in the "real" function
+    // scope, but can't be resolved from it
+    outer_scope()->AllocateStackSlot(var);
+  } else if (is_block_scope()) {
     DeclarationScope()->AllocateStackSlot(var);
   } else {
     var->AllocateTo(Variable::LOCAL, num_stack_slots_++);