Don't require individual source classes in sandcastle app

[hpinkos]

No description provided.

[cesium-concierge]

Thanks for the pull request @hpinkos!

• ✔️ Signed CLA found.
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  • If this change updates the public API in any way, please add a bullet point to CHANGES.md.
• ❔ Unit tests were not updated.
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Reviewers, don't forget to make sure that:

• Cesium Viewer works.
• Works in 2D/CV.
• Works (or fails gracefully) in IE11.

[hpinkos]

@OmarShehata please review

[OmarShehata]

It would have been helpful to link to #7523 for context in the PR description.

While I think the way Sandcastle is structured now is still incredibly confusing (the built version relies on the require'd Cesium failing to load, and loading it from window.Cesium, which comes from somewhere else?) this looks like an improvement if the built Sandcastle doesn't break when it can't access Source/.

[hpinkos]

Oh, i didn't realize we had that issue written up. thanks @OmarShehata

[mramato]

@OmarShehata if you ever really wanted an awesome side project, rewriting Sandcastle is probably number 2 on the list of longstanding giant undertakings (ES6 is number 1).

I'm actually a little concerned about how easily we'll be able to update Sandcastle for ES6 Cesium, so the issues are slightly related. Might be a good discussion to have at our code sprint, even if we can't take on the challenge this time around, preparing a roadmap issue with a full breakdown of doing it would be a big help.