`isStream` should be flagged as a reserved property on Components

[noslouch]

See this twiddle for a reduced test case. Open the console to see the error message:

Uncaught TypeError: object.subscribe is not a function

If you set isStream as a property on a Component (at least, maybe other classes too), rendering borks because isStream is used internally. I can put together a failing test and possibly a PR, just wanted to note it here in case anyone else wants to pick it up.

I'm imagining a helpful error message at run time that just states that isStream is a reserved property name.

[mmun]

We should not be reserving isStream, instead we should change this branding to a symbol to avoid interfering.