Fix super/this capture for extends null

[rbuckton]

Fixes #13276

[mhegazy]

We will need to port this to release-2.1 as well.

[DanielRosenwasser]

I would consider an additional check, which is that if the extends clause element is syntactically null, no capture should be performed at all. Any thoughts?

[DanielRosenwasser]

Also, I'd consider just making it _super && instead of _super !== null &&.

[rbuckton]

The spec is explicit about null. undefined should be an error/undefined behavior.

[mhegazy]

@rbuckton can you merge this and port it to release-2.1 as well.

[rbuckton]

Sure. I was just updating it based on Daniel's suggestion above.

[rbuckton]

I've merged this into release-2.1 as well.