Fix tab indicator animation so it doesn't snap to the previous tab.
Fix tab indicator animation so it doesn't snap to the previous tab.

If you tap multiple tabs in a row, the tab animation used to snap to the
last selected tab when starting a new animation. Fix that.

Also use the BuilderTransition so we don't have to rebuild the tab bar
every frame.

cbracken/flutter,aghassemi/flutter,flutter/flutter,tvolkert/flutter,turnidge/flutter,cbracken/flutter,tvolkert/flutter,jason-simmons/flutter,flutter/flutter,Hixie/flutter,tvolkert/flutter,flutter/flutter,Hixie/flutter,cbracken/flutter,flutter/flutter,turnidge/flutter,aghassemi/flutter,tvolkert/flutter,tvolkert/flutter,tvolkert/flutter,turnidge/flutter,cbracken/flutter,Hixie/flutter,turnidge/flutter,cbracken/flutter,flutter/flutter,Hixie/flutter,jason-simmons/flutter,Hixie/flutter,flutter/flutter,tvolkert/flutter,flutter/flutter,jason-simmons/flutter,Hixie/flutter,Hixie/flutter,flutter/flutter,cbracken/flutter,jason-simmons/flutter,flutter/flutter,cbracken/flutter,tvolkert/flutter,Hixie/flutter,cbracken/flutter,Hixie/flutter,aghassemi/flutter,Hixie/flutter,tvolkert/flutter,cbracken/flutter,turnidge/flutter,aghassemi/flutter,flutter/flutter,cbracken/flutter,tvolkert/flutter