Adds the possibility of setting a Code object as the callback of a FunctionTemplate.

src/api-natives.cc

Index: src/api-natives.cc
diff --git a/src/api-natives.cc b/src/api-natives.cc
index d8dd1510417641197da707cf27f79e51586886d3..ea6d6bf46f886ce40a1d9e5a18bc52a0d3e1c4ca 100644
--- a/src/api-natives.cc
+++ b/src/api-natives.cc
@@ -438,7 +438,13 @@ void ApiNatives::AddNativeDataProperty(Isolate* isolate,
 Handle<JSFunction> ApiNatives::CreateApiFunction(
     Isolate* isolate, Handle<FunctionTemplateInfo> obj,
     Handle<Object> prototype, ApiInstanceType instance_type) {
-  Handle<Code> code = isolate->builtins()->HandleApiCall();
+  Handle<Code> code;
+  if (obj->call_code()->IsCallHandlerInfo() &&
+      CallHandlerInfo::cast(obj->call_code())->fast_handler()->IsCode()) {
+    code = isolate->builtins()->HandleFastApiCall();
+  } else {
+    code = isolate->builtins()->HandleApiCall();
+  }
   Handle<Code> construct_stub = isolate->builtins()->JSConstructStubApi();
 
   obj->set_instantiated(true);