async_wrap: add GetAsyncId() method

Allow handles to retrieve their own uid's by adding a new method on the
FunctionTemplates. Implementation of these into all other classes will
come in a future commit.

src/async-wrap.cc

@@ -315,6 +315,14 @@ AsyncWrap::~AsyncWrap() {
 }
 
 
+void AsyncWrap::GetAsyncId(const FunctionCallbackInfo<Value>& args) {
+  AsyncWrap* wrap;
+  args.GetReturnValue().Set(-1);
+  ASSIGN_OR_RETURN_UNWRAP(&wrap, args.Holder());
+  args.GetReturnValue().Set(wrap->get_id());
+}
+
+
 Local<Value> AsyncWrap::MakeCallback(const Local<Function> cb,
                                      int argc,
                                      Local<Value>* argv) {

src/async-wrap.h

@@ -85,6 +85,8 @@ class AsyncWrap : public BaseObject {
                          v8::Local<v8::Value> unused,
                          v8::Local<v8::Context> context);
 
+  static void GetAsyncId(const v8::FunctionCallbackInfo<v8::Value>& args);
+
   static void DestroyIdsCb(uv_idle_t* handle);
 
   inline ProviderType provider_type() const;