Allow local-running Cesium to load its resources

[cguinnup]

Fixes #5830

[cesium-concierge]

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[ggetz]

Hi @pizza2code , thanks for the PR!

So this issue of using the file protocol with Cesium has come up before, see #5326 and #3935

Instead of this change, we'd reccomend using something like Electron to run your app. See our blog post on getting that set up here: https://cesium.com/blog/2016/04/04/an-introduction-to-cesium-desktop-apps-with-electron/

[ggetz]

@hpinkos Can we consider reopening this?
See #5830 (comment).

Sorry for the additional noise.

Also, per that comment, we can skip the localFile check and just check for xhr.status === 0 in the onload function.

[shunter]

Actually, I think checking for file protocol will still be necessary. From reading the spec, "0" would appear to be a valid error indication for a network-level error for HTTP requests.

[ggetz]

OK, thanks @shunter !

[mramato]

@pizza2code we can't merge this until you send in a CLA (as noted above). Since this is a small change, if you are unable or don't want to fill out a CLA, I can make the change myself in a different PR. Just let us know either way. Thanks for the PR either way!

[cguinnup]

@mramato Sorry, not sure I'll be able to sign the CLA due to clauses in my employment agreement. You can go ahead and make the change (you're right, it's pretty small) in another PR. :)

[hpinkos]

Thanks @pizza2code! I've opened a PR to add the changes here: #5892
Thanks for bringing this up