[url_launcher]fix issue #25721 url_launcher: reset _currentSession only at end of browsing instead of after VC display

[MagicalTux]

this solves issue flutter/flutter#25721 by having completion code called when user presses the "Done" button (safariViewControllerDidFinish, which calls dismissViewControllerAnimated), allowing closeWebView() to work in iOS.

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[MagicalTux]

I signed it!

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CLAs look good, thanks!

[timtraversy]

I made a similar PR for this problem at #997. Whatever one we go with this should get merged so the function can work on iOS.

[MagicalTux]

@timtraversy

I made a similar PR for this problem at #997. Whatever one we go with this should get merged so the function can work on iOS.

Sorry I didn't see it, as I was primarily looking at bug reports in https://github.com/flutter/flutter. Hopefully either one can be merged, in the meantime I'm using a modified version locally.

Also, I created another issue regarding using safariVC to authenticate users, see flutter/flutter#25928 - I guess you may have similar needs and/or requirements

[cyanglaz]

The same issue seems has been fixed by 36c56ee
I am closing the PR now. If you find the PR is needed for different issues, please feel free to reopen it.