This makes Fetch rely less on using main_thread_vm().current_realm(),
which relies on the dummy execution context if no JavaScript is
currently running.
With so much infrastructure implemented, we can finally add the last
piece of this puzzle - the fetch() method itself!
This contains a few hundred lines of generated code as handling the
RequestInfo and RequestInfo parameter types manually is not feasible,
but we can't use the IDL definition as the Window object is handwritten
code at the moment.
It's neatly tucked away in Bindings/ and will be removed eventually.
