Speech Api in flutter web

Hi, I want to add Google speech to text API. In flutter web.
Live Speech to text. I'm figuring it out.
Can you help me, and

one more thing when I tried to create an HTTP client. it's returning null.

final _googleSignIn = GoogleSignIn(
scopes: [SpeechApi.cloudPlatformScope],
);

var httpClient = (await _googleSignIn.authenticatedClient())!;