firebase-auth-2-step-4.dart

void loginWithGitHub(String code) async {
  //Step 4
  final response = await http.post(
    "https://github.com/login/oauth/access_token",
    headers: {
      "Content-Type": "application/json",
      "Accept": "application/json"
    },
    body: "{\"client_id\":\"" + CLIENT_ID
        + "\",\"client_secret\":\"" + CLIENT_SECRET
        + "\",\"code\":\"" + code
        + "\"}",
  );

In the body interpolation the dart string can be single quote string, so you wouldn't have to escape the quotes inside the string everywhere => it'd make the code more readable. (Unless you are using some automatic styling plugin which strictly enforcing the double quotes.)
I'd also use string interpolation for the variables:

body: '{"client_id":$CLIENT_ID",' + 
        + '"client_secret":"$CLIENT_SECRET",' + 
        + '"code":"$code"}',