getDevices method
should return a promise (which will be converted to a future using JSUtils.promiseToFuture) with a list of Objects. These objects should then be able to be converted to a WebBluetoothDevice using WebBluetoothDevice.fromJSObject.
NOTE: Currently no browser supports this without a flag needing to be set!
See:
Implementation
Object getDevices() => _NativeBluetooth.getDevices();