This is Part 1 of the Dart and Flutter series—practical guides, architectural deep dives, and hard-earned engineering lessons from the field. Each article is completely standalone. Picture this familiar scene: You’re deep in the zone. You’re practicing disciplined Test-Driven Development (TDD). You write a failing test, write the minimal code to satisfy it, and refactor. Red, green, refactor. Your fingers are flying. Then, you add a single new method to your UserRepository inter...