Tracer: display of values created by builtins list, is_... does not show anything

[martin-henz]

pair(list(1), pair(1, null));

leads to

[[1, ], [1, null]];

Note that null is displayed when explicitly created with null keyword, but null created with the list builtin is not displayed.

[martin-henz]

This bug is marked critical because it will be quite prominent in the teaching of CS1101S.

[kavishsathia]

I think this is happening because we're not setting the raw value at line 92 of lists.ts.

Should be return new StepperLiteral(null, 'null') instead of return new StepperLiteral(null). I’ll test it and open a PR when I have time, but feel free to take it on if you get to it first.

[martin-henz]

We seem to have a similar problem with true and false: