Check binding exists

[kangax]

Ran into this in one of our files when NOT using es2015 transform. I'll try to make a test case but don't have one for now.

[boopathi]

Related: babel/babel#4815 . So we should probably include a scope.crawl before running DCE.

I guess a for statement using let in the init and an arrow function (referencing this let binding) inside the for loop might fail. (this is true for es2015. misread the desc)

[boopathi]

I had similar issues in mangler as well, where a BindingIdentifier has no binding - and I reduced it to this ... without clearing the traverse cache and re-crawling the entire scope that will throw.

[kangax]

@boopathi so what do you think we should do here? Drop this PR in favor of #252?

[boopathi]

No. Suggesting we should do traverse.clearCache and program.scope.crawl() before anything in DCE, and check if that fixes the bug.

This check really is a work around where we have a BindingIdentifier(here a function parameter) and ignore it when we do not have a binding in the scope.

[boopathi]

I didn't come across any other case where a binding is missing at obvious places. So, a snippet that reproduces this problem should be helpful to debug if we missed anything.

[kangax]

I just ran into this again, while running over Nuclide code base. It seems to only happen when there's other plugins that transform code before Babili (https://github.com/facebook/nuclide/blob/master/pkg/nuclide-node-transpiler/lib/NodeTranspiler.js#L43-L75); transforming source code itself didn't exhibit the issue...