invalid_visibility_annotation

The member '{0}' is annotated with '{1}', but this annotation is only meaningful on declarations of public members.

The analyzer produces this diagnostic when either the visibleForTemplate or visibleForTesting annotation is applied to a non-public declaration.

The following code produces this diagnostic:

import 'package:meta/meta.dart';

@visibleForTesting
void _someFunction() {}

void f() => _someFunction();

If the declaration doesn't need to be used by test code, then remove the annotation:

void _someFunction() {}

void f() => _someFunction();

If it does, then make it public:

import 'package:meta/meta.dart';

@visibleForTesting
void someFunction() {}

void f() => someFunction();