[html] Fix `.onBeforeUnload` memory leaks

This code was added in 2013 for what looks like the purpose of
aligning functionality across browsers.
https://codereview.chromium.org/12218111

The custom event stream provider used to wrap events is missing
any cleanup code. The interesting part (returning `.returnValue`
on the wrapped event) was removed in 2016.
https://codereview.chromium.org/1894713002

Since then it appears that the custom event stream provider no
longer provides any use.

Canceling the stream subscription that is returned from
`window.onBeforeUnload.listen()` was not actually removing the
listener on the underlying native element. The listener would
remain and its callback was never garbage collected. This is
especially a problem in apps that correctly clean up when
destroyed. After multiple hot restarts and cleanup executions,
multiple listeners and callbacks remained all holding memory
and keeping referenced elements alive.

Change-Id: I34ae78c95b6dc85eaf2b398a75b0c55a7a7bc047
Fixes: https://github.com/dart-lang/sdk/issues/48399
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/233021
Reviewed-by: Srujan Gaddam <srujzs@google.com>
Commit-Queue: Nicholas Shahan <nshahan@google.com>