cast_from_null_always_fails

This cast always throws an exception because the expression always evaluates to 'null'.

The analyzer produces this diagnostic when an expression whose type is Null is being cast to a non-nullable type.

The following code produces this diagnostic because n is known to always be null, but it's being cast to a non-nullable type:

void f(Null n) {
  n as int;
}

Remove the unnecessary cast:

void f(Null n) {
  n;
}

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