JS: Allow MaD flow through properties

[asgerf]

Adds the possibility of stepping through a property read obj -> obj.foo using a summary CSV row where:

• The input part is the empty string, and
• The output part is Member[foo].

@RasmusWL what do you think?

[RasmusWL]

Nice, looks good to me 👍

I assume that using Member[foo,bar], adding flow from obj to obj.foo and obj.bar, would also work?

[asgerf]

I assume that using Member[foo,bar], adding flow from obj to obj.foo and obj.bar, would also work?

Yes, that should work as well.

But you can't do Member[foo].Member[bar] because the simple approach I took here won't work when the .foo and .bar operations occur in different functions. For that to work we'd have to interpret the initial .foo as a call to a function with an output spec of ReturnValue.Member[bar].

[asgerf]

After ruminating on this some more, I think the more general solution is to add something like AccessorCall[x] which models an access to the x attribute as a call (even if it's not considered a call in the language IR).

Then a step like foo -> foo.bar could be modelled as

package;type;Member[foo].AccessorCall[bar];Argument[self];ReturnValue

This avoids the somewhat awkward special case where the output must consist of a single token.

To be clear, with the solution currently in this PR it would look like this:

package;type;Member[foo];;Member[bar]

@aschackmull do you have any thoughts on this? The context is that we want to be able to model flow through things that aren't calls, but in principle could be modelled as a calls. It looks like the two approaches are:

• Add a special rule for specifying a step through a member accesses. This can be implemented a step, without a flow summary.
• Model member access as a call, even though it's not a call in the language IR. I'm still not familiar enough with the flow summary code to foresee what problems this will incur. (JS and Python don't have flow summaries at the moment, and the question is moot for Ruby since everything is a method call there).