The documentation for Dart and (especially) Flutter are fantastic. It's clear that Google has put a great deal of time into the entire ecosystem. If you need a quick reference, or additional examples to compliment Flutter by Example, you can find Dart documentation at dart.dev and Flutter documentation at flutter.dev.

Pub

Pub is Dart's dependency management system and library. If you're looking for an open-sourced library to solve a problem for any Dart (and Flutter) program, you'll find there.