Maintain this context when sequence is callable

[nathancahill]

Fixes #1722

[lukastaegert]

Hi @nathancahill, this looks really nice, thank you a lot! I have thought about it and I agree, the only situation where this can hurt us is where the sequence expression is a direct callee of a call expression, just how you implemented it.

On a side note, this also implies that when we simplify conditional and logical expressions (at least the first case can actually happen, though not as prominently), we need to keep the this context of expressions in mind as well. This is certainly beyond the scope of this fix and might warrant a separate issue. At that point, we should also revisit how those expressions interact with each other.

One thing I was wondering about if it would make sense to add a function test besides the form tests you added that actually checks that the member expression uses the right context. For instance, function tests can mock an imported function that receives the current this context and throws an error if it is not the global object. However, this would only serve some documentary purpose to explain this uncommon usage of the comma operator to future rollup contributors. What do you think?

[nathancahill]

Sounds good, I'll add a test for that.

[nathancahill]

How does that look?

[lukastaegert]

Awesome! Even simpler than what I had in mind 😜