Flutter is an open-source UI software development kit created by Google. It is used for developing platform-agnostic applications for Android, iOS, Linux, Mac and Windows.
The easiest way to use beacons with Flutter is to use a ready-made library. Flutter Gems is a curated list of 5600+ useful Dart & Flutter packages that are categorised based on functionality. They have a section for Bluetooth, NFC, Beacon packages.