How do I access flutter dart-defines at native level? - flutter

Question

I am developing a flutter desktop app for windows. I want to use dart-defines to configure the text shown in the window title. To do it in Dart code is not a problem, but thats too late, because the window title is already set in the native c++ part, before the Dart code executes.
I start my application with flutter run --dart-define=APP_TITLE=AwesomeApp. If I access the environment variable APP_TITLE within Dart code it works as I expect:
const appTitle= String.fromEnvironment('APP_TITLE', defaultValue: 'Not Found');
debugPrint("App Title: " + appTitle); // -> App Title: AwesomeApp
But if I try to access the environment variable from the main.cpp where the window title is defined, I cannot access it.
#include <iostream>
#include <cstdlib>
std::string getVariable(std::string const& name) {
size_t size;
getenv_s(&size, nullptr, 0, name.c_str());
if (size > 0) {
char* tmpvar = new char[size];
errno_t result = getenv_s(&size, tmpvar, size, name.c_str());
std::string var = (result == 0 ? std::string(tmpvar) : "");
delete[] tmpvar;
return var;
}
return "";
}
int APIENTRY wWinMain(_In_ HINSTANCE instance, _In_opt_ HINSTANCE prev,
_In_ wchar_t *command_line, _In_ int show_command) {
...
std::string appTitle = getVariable("APP_TITLE");
if(appTitle != "")
std::cout << "App Title: " << appTitle << std::endl;
else
std::cout << "App Title: " << "Not Found" << std::endl; // -> App Title: Not Found
...
}
For most usecases access of environment variables within Dart code is sufficient. But the window title need to be defined in the native main.cpp optimally.
Am I doing a mistake or is it just not possible to access the dart-defines at native level? What is the suggested way of configuring a flutter desktop application for different envrionments like dev or prod.