Add missing static resource.

[gspencergoog]

This adds a missing asset to the resources manifest.
I didn't know I needed to run pub run grinder index-resources when I added it...

[devoncarew]

@gspencergoog, is this timely? @jcollins-g is out until next wednesday

[gspencergoog]

Well, without it, the play buttons will be missing on animations on the Flutter doc site. There aren't a lot of them yet, but I'd like to fix it.

[devoncarew]

ok, gotcha.

I think to get travis green, you'll need to run grind update-test-package-docs (as per https://github.com/dart-lang/dartdoc/blob/master/CONTRIBUTING.md).

[devoncarew]

I didn't know I needed to run pub run grinder index-resources when I added it...

We should likely add a travis check for that, or in-line generation into something else, like update-test-package-docs. /cc @jcollins-g

[gspencergoog]

Yes, a Travis check to make sure that the resource manifest doesn't change if you run grind index-resources would be good.

[gspencergoog]

Thanks for the review @devoncarew! (I don't have write access, so you'll have to commit it for me... :-))