Remove cached references to Node constructor

[ekashida]

This issue provides context for why we cache Node references along with guidance on when we can remove the related code.

There is legacy code in the platform, for which the owning team is unknown, that sets window.Node to null during the unload event, presumably to deal with IE memory leaks. When LWC component authors subsequently invoked certain polyfilled DOM APIs (e.g., composedPath(), event.target, etc) during the unload event, it caused instanceof Node checks to throw exceptions. Rather than taking on the deceptively monumental task of removing that line of code, we decided to simply cache the Node reference internally.

These changes can be safely removed when SfdcInitialize.js is updated to remove the window.Node = null.