How do I build different versions of my Flutter app for qa/dev/prod?

Building on Seth's idea, here's an example that sets up a global representing the BuildEnvironment named env.

env.dart

import 'package:meta/meta.dart';

enum BuildFlavor { production, development, staging }

BuildEnvironment get env => _env;
BuildEnvironment _env;

class BuildEnvironment {
  /// The backend server.
  final String baseUrl;
  final BuildFlavor flavor;

  BuildEnvironment._init({this.flavor, this.baseUrl});

  /// Sets up the top-level [env] getter on the first call only.
  static void init({@required flavor, @required baseUrl}) =>
      _env ??= BuildEnvironment._init(flavor: flavor, baseUrl: baseUrl);
}

main_dev.dart

import 'package:flutter/material.dart';
import 'env.dart';
import 'app.dart';

void main() {
  BuildEnvironment.init(
      flavor: BuildFlavor.development, baseUrl: 'http://dev.example.com');
  assert(env != null);
  runApp(App());
}

main_prod.dart

import 'package:flutter/material.dart';
import 'env.dart';
import 'app.dart';

void main() {
  BuildEnvironment.init(
      flavor: BuildFlavor.production, baseUrl: 'http://example.com');
  assert(env != null);
  runApp(App());
}

• import env.dart to expose the env variable.

• run and build the app using the target option.

  flutter run -t lib/main_dev.dart flutter build -t lib/main_dev.dart

To integrate with VS Code, define launch configurations:

.vscode/launch.json

{
  "version": "0.2.0",
  "configurations": [
    {
      "name": "development",
      "program": "lib/main_dev.dart",
      "request": "launch",
      "type": "dart"
    },
    {
      "name": "production",
      "program": "lib/main_prod.dart",
      "request": "launch",
      "type": "dart"
    }
  ]
}

I had originally set out to use command line arguments passed to Dart's main function, but I don't think args can currently be passed on the command line with flutter run or flutter build, although VS Code and Android Studio both support passing args to main. It also seems build flavor as a command line arg to main is not appropriate since args can be passed after the build process.

Release and debug mode can now be acquired using

const bool isProduction = bool.fromEnvironment('dart.vm.product');

Because this is a constant it works with tree-shaking.
So code like

if(isProduction) {
  // branch 1
} else {
  // branch 2
}

would only include one of these two branches into production code depending on isProduction