use null prototype for opaque wrappers

[anonrig]

This is important for passing several web-platform tests related to streams and fetch.

We use null prototype to prevent Object.prototype.then patches from intercepting internal promise operations (required for WPT compliance).

[jasnell]

An alternative way of doing this that might be better, is to add the following when the opaqueTemplate is created:

auto protoTemplate = v8::FunctionTemplate::New(ptr, &throwIllegalConstructor);
protoTemplate->RemovePrototype();
opaqueTemplate->SetPrototypeProviderTemplate(protoTemplate);

This would avoid the added branch and after-creation prototype mutation in the hotpath and allows the wpt's to pass. I haven't benchmarked to see if it actually is faster tho but it's worth an experiment.

[anonrig]

@jasnell I believe you're right and I think it is actually faster than the current state.

[jasnell]

LGTM. I do think it might be beneficial for either the PR or commit message to explain a bit about WHY this fixes the tests as it's non-obvious, but that's not necessarily blocking.