Cljs 1.10.x worker target support

[milt]

As of clojure/clojurescript@e380903, ClojureScript is getting a proper compiler :target option for WebWorkers: :webworker.

This is great, as it means the hacky process I outlined here will no longer be necessary, since worker builds with :optimizations :none can now set a :main. The flipside is that figwheel needs to expect such cases during pre-compilation steps and script injection.

This pull:

• Adds :webworker to the valid targets in the spec
• Shadows the node checks during injection for workers
• Addresses an odd behavior where the feature? check during the definition of local-persistent-config throws a runtime error. Since this check is present in previous versions of figwheel that work fine with workers, and the same check runs fine inside a function, I'm guessing it has something to do with changes to the cljs compiler, so my fix for this should be regarded with suspicion.

I realize that this might be a little premature given how new the worker support in Cljs is, but at the very least I hope it is informative! Thanks again for figwheel!

[milt]

As I mentioned in the description, I don't understand why feature? doesn't work on its own here as it has in the past. Explicit checking for html-env? seems naive, since it might be provided by a polyfill/lib in non-browser environments.

[milt]

Ah, just took a look at #638

My original solution for the localStorage issue was definitely bad.

[milt]

OK, figured it out! the feature? function closed over cljs.core/exists? meaning that in environments without the global object in question defined, a ReferenceError is thrown before the check runs. in #638 @boogie666 was dealing with an empty but defined object so it was sufficient, but in a worker (and probably elsewhere) localStorage is undefined, so we need to pass that through to exists? before evaluation.

As usual, the solution to a macro problem turns out to be more macros.

[bhauman]

This seems like it would work really well. And since it's additive there are no backwards compatability problems.

I'm assuming you have tested this and it works fine?

[milt]

Sorry, was out of the loop for a few days! Yeah, I've tested it in the browser, a worker, and node, don't currently have react-native set up.
Thanks again!