Incorrect interceptors on patched elements in Firefox

[blois]

With custom elements and the polymer.js polyfill, HTMLElements are getting interceptors for HTMLUnknownElement and breaking interceptor checks for later elements.

Basically unknown custom elements in Firefox are getting their prototype swizzled to HTMLElement, but the toString is still resulting in HTMLUnknownElement, so HTMLElement prototype is getting HTMLUnknownElement's interceptor.

[rakudrama]

A work-around for this is to call getInterceptor on a known direct instance of HTMLElement.

A fix would be to patch some or all available prototypes a isolate start-up.
This would have the advantage of reducing map transitions in V8.

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Added this to the M7 milestone.
Removed Priority-Unassigned label.
Added Priority-Medium label.

[rakudrama]

Marked this as being blocked by #13218.

[kasperl]

Removed this from the M7 milestone.
Added this to the M8 milestone.

[rakudrama]

Pete, can you verify that pre-patching is working?
You should be able to remove the work-around

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Set owner to @blois.
Added Fixed label.