ffi_native_must_be_external

Native functions must be declared external.

The analyzer produces this diagnostic when a function annotated as being @Native isn't marked as external.

The following code produces this diagnostic because the function free is annotated as being @Native, but the function isn't marked as external:

import 'dart:ffi';

@Native<Void Function(Pointer<Void>)>()
void free(Pointer<Void> ptr) {}

If the function is a native function, then add the modifier external before the return type:

import 'dart:ffi';

@Native<Void Function(Pointer<Void>)>()
external void free(Pointer<Void> ptr);