Proposal: Well-known symbol for DOM shim about community-protocols

[matthewp]

@fgirardey talked about this issue in #17 and said:

I've read #18 but I don't see how it will help because the existing custom elements will need to opt-in and use those well-known symbols.

The idea here is for libraries to use the well-known symbol, not for component authors to do so. If libraries were to adopt this symbol that would mean that most components, which generally do not need the browser globals, would also work without global shims.

There will always be some low-level custom elements that do need browser globals, this issue isn't an attempt to provide full compatibility with every element that exists in the wild. Although if this became a community standard which library authors adopted, there's no reason why low-level elements couldn't also do so.