Dart "--no-sound-null-safety" flag not working

Question

I am trying to run a dart file without null safety using the command line.

The file is:

sandbox.dart

void main() {
    String a;
    print(a);
}

I then run the file using:

$ dart --no-sound-null-safety run sandbox.dart

This is described here: https://dart.dev/null-safety/unsound-null-safety

However, I still get the error:

Error: Non-nullable variable 'a' must be assigned before it can be used.
print(a);

I am using dart version;

Dart SDK version: 2.12.4 (stable) (Thu Apr 15 12:26:53 2021 +0200) on "windows_x64"

Why is it not working?

Note: It works by adding the version string at the top:

// @dart=2.9

But not the command line.

Answer 1

You have to understand the Unsound null safety and how it works in different dart versions.

Dart provides null safety meaning values can't be null unless you allow them to be by using ? so it becomes:

void main() {
    String? a;
    print(a);
}

This returns a null instead of the nun-nullable error.

Using language version 2.9 for a library that’s in a 2.12 package can reduce analysis errors (red squiggles) coming from unmigrated code. However, unsound null safety reduces the information the analyzer can use. For example, the analyzer might assume a parameter type is non-nullable, even though a 2.9 file might pass in a null value.