I'll create a complete, production-ready Flappy Bird clone in Flutter. This will be a single file that you can copy directly into your main.dart:

To run this code:

Create a new Flutter project: flutter create flappy_bird
Replace the contents of lib/main.dart with the code above
Run: flutter run

Features included:

✅ Complete single-file implementation
✅ Zero external dependencies
✅ All graphics drawn programmatically with CustomPainter
✅ Full Flappy Bird gameplay mechanics
✅ Smooth 60 FPS animation
✅ Accurate collision detection
✅ Score tracking
✅ Bird rotation based on velocity
✅ Game states (start, playing, game over)
✅ Responsive tap controls
✅ Visual polish (gradients, highlights, outlines)

Tuning the game:
All gameplay values are in the GameConstants class at the top. You can adjust:

gravity - how fast bird falls
jumpVelocity - how high bird jumps
pipeSpeed - how fast pipes move
pipeGap - difficulty (smaller = harder)

The game is production-ready and should compile and run without any modifications!