Cesium: Use ES6 "class" as module syntax

[worlddai]

So I can extend your class directly using extends,and so on.

[OmarShehata]

This is to support browsers that don't yet support ES6 syntax without requiring a transpilation step. Here's a quote from our blog post going over the whole migration that I encourage you to check out:

Even though we’ve moved to ES6, the combined Cesium.js file still supports browsers that lack ES6 support, such as IE 11. As long as your build system can target IE 11, Cesium ES6 modules will continue to work without requiring additional transpilation.

With that said, having a transpilation is something we want to support in order to use newer ES6 syntax and features. Also from the blog:

Since Chrome, Firefox, and Safari already support some of the latest JavaScript features, we should be able to keep our minimal build during development policy in place and start to leverage newer features such as async/await/Promise/generators/etc… We would still need to run the final output through Babel as part of the build process, but Rollup already has good support for this step.

I'm going to leave this open as a feature request to use this particular ES6 feature, since even if we do add a transpilation step it would have to be another effort to change how classes are declared.

[mramato]

@OmarShehata I'm actually going to close this, the use of class will be part of a much larger discussion/decision/change so having this open as a standalone issue isn't really actionable.

[thw0rted]

@mramato is there a different (good) place to discuss this publicly? I'm currently consuming Cesium from Typescript (c.f. #5717 #4434 etc) and I think I could fully automate that process if the "class-like" new-able function pattern that Cesium uses were to move to ES2015 class syntax. I explain in the "Typings" issue, but briefly, Typescript can successfully extract type information from JSDoc for almost all of Cesium, except where getter/setter functions are passed to Object.defineProperties and decorated with @memberof.

As a small test case, I converted Viewer.js by hand, which took about 5 minutes of manual copy/pasting. Most of the work was converting the getter/setter syntax from foo: { get: function(){...}, set: function(...) { ... } } to get foo() {...}; set foo(...) { ... };. I haven't looked at automating this yet but I believe I can at least make the manual process pretty straightforward. After making this conversion, the Typescript output (.d.ts files) is correct, and the test suite still passes.