prefer_asserts_with_message

Missing a message in an assert.

The analyzer produces this diagnostic when an assert statement doesn't have a message.

The following code produces this diagnostic because there's no message in the assert statement:

void f(String s) {
  assert(s.isNotEmpty);
}

Add a message to the assert statement:

void f(String s) {
  assert(s.isNotEmpty, 'The argument must not be empty.');
}