Consider adding "why promoted" context information for warnings/errors caused by type promotion

[stereotype441]

Inspired by this comment: dart-lang/language#1523 (comment)

Consider the following code:

f(Object? x) {
  g(x as int); // (1)
  if (x == null) { // (2)
    print('x was null');
  }
}
g(int? x) {}

Note that the user migrated this code incorrectly from a pre-null-safety version: at (1), they should have used x as int? to preserve the behavior that null is allowed.

Currently the analyzer generates the following warning at the line marked (2):

   info • test.dart:3:9 • The operand can't be null, so the condition is always false. Try removing the condition, an enclosing condition, or the whole
          conditional statement. • unnecessary_null_comparison

Based on this warning, it's not easy to figure out that the mistake was in the as expression at (1). A possible way to help this would be to add context information to the warning explaining the reason the operand can't be null: because it was promoted at line (1).

It should be possible to do this in a similar way to how we are adding "why not promoted" context information (#44897).

[a14n]

Note that there's the lint cast_nullable_to_non_nullable to catch (1)