build.yaml

targets:
  $default:
    sources:
      - "node/**"
      - "lib/**"
    builders:
      build_node_compilers|entrypoint:
        generate_for:
        - node/**
        options:
          compiler: dart2js
          # List any dart2js specific args here, or omit it.
          dart2js_args:
          - --checked

index.dart

import 'package:firebase_functions_interop/firebase_functions_interop.dart';
import 'package:node_io/node_io.dart';

import 'dart:convert';

void main() {
  functions['helloWorld'] =
      FirebaseFunctions.https.onRequest(helloWorld);
}

void helloWorld(ExpressHttpRequest request) {
  request.response
    ..headers.contentType = ContentType.JSON
    ..write(json.encode({'key' : 'hello from dart'}))
    ..close();
}

init.sh

mkdir project
cd project
firebase init functions