MessageEvent location href is inaccessible

[nex3]

Currently there's no way to get the href of the source location of a MessageEvent. This is very important for applications that exchange messages with multiple iframes that have the same origin; without it, there's no way to tell which frame sent which message.

A reproduction is available at https://github.com/nex3/post_message_test.

See also issue #21747.

[natebosch]

It looks like the _LocationCrossFrame has a setter for href but not a getter.

I would imagine it should be safe to add a getter then, but we've also moved away from dart:html for the use case that was impacted.