Forward mouseup and mousedown too.
Forward mouseup and mousedown too.

In my application, I detect clicks on the backdrop by comparing the `MouseEvent`'s x and y coordinates to the dialog's box dimensions and closing the dialog if the user clicked on the backdrop.

However I can't simply use the 'click' event because my dialogs are resizable (using the browser native resize grip in the bottom-right corner of the dialog). If a user drags the resize handle to make the dialog bigger - and if the dialog has a `max-width` and/or `max-height` and the user keeps on dragging the mouse cursor and releases their mouse button over the backdrop then the 'click' event is raised with the `MouseEvent` coordinates over the backdrop - so I use the mouseup and mousedown events to detect if the mouse moved between the backdrop and the dialog-proper (again, by comparing coordinates of the MouseEvent with the size of the dialog-proper), and based on that it either ignores the click or it closes the dialog.

My solution works with this Polyfill library, but I first needed to invoke the `backdropClick_` function for `mouseup` and `mousedown` events - so this is me contributing those trivial changes back.
GoogleChrome/dialog-polyfill,GoogleChrome/dialog-polyfill