[scoped-custom-element-registry] Integrate and test with custom-elements polyfill

[kevinpschaaf]

Task: The scoped-custom-element-registry polyfill is currently only tested to work on browsers implementing the native web components specs (Custom Elements & Shadow DOM). This task is to investigate and make changes necessary to integrate and test the scoped registry polyfill when loaded on top of the custom-elements polyfill.

It is expected that code such as that using Reflect.construct may need to have alternate implementations to work on older browsers (and it's likely that the code paths for working on polyfilled custom-elements can be made simpler, since they can generally assume compiled ES5 classes where the constructor can simply be applied to any instance.

[stale[bot]]

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