Add shared_preferences package

[krjw-eyev]

I have used shared_preferences a lot lately and it would be great to test small thing with it via dartpad. Since shared_preferences supports web I think this should be a great addition to the already supported packages.

[domesticmouse]

I'd wonder if this will work in the sandbox iframe. It may work for a session and then disappear, or not work at all.

[johnpryan]

Closed in #995

[domesticmouse]

Actually, this isn't shipped to production, so I'm re-opening this issue. This package has been shipped to our hidden testing instance https://dartpad.dev/?channel=dev

Please exercise your suggested package on this instance and confirm here that it works as intended. If you do confirm this as working I'll ship it to stable, beta, master, and old channels on dartpad.dev. If you don't confirm, I won't ship it.