Use internal mutability for Event.
Use internal mutability for CustomEvent.
Use internal mutability for UIEvent.
Use internal mutability for EventTarget.
Use internal mutability for HTMLIFrameElement.
Use internal mutability for Node.flags.
Stop mutating Element::namespace.
Use internal mutability for CharacterData::data.
Use internal mutability for Window::browser_context.
Use internal mutability for Window::{active_timers, next_timer_handle}.
vtable cleanup