Update to GWT 2.8.0

[jschmied]

Add stubbing for ValueListBox Fix assertion in ValueBox.

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[jschmied]

I signed it!

[googlebot]

We found a Contributor License Agreement for you (the sender of this pull request), but were unable to find agreements for the commit author(s). If you authored these, maybe you used a different email address in the git commits than was used to sign the CLA (login here to double check)? If these were authored by someone else, then they will need to sign a CLA as well, and confirm that they're okay with these being contributed to Google.

[jschmied]

All commits were authored by me.

[googlebot]

CLAs look good, thanks!