Floating Action Button transitions

When the scaffold's floating action button (FAB) is replaced the old FAB shrinks to zero and then the new one grows to its full size. When a FAB is shown, its child spins into place.

This is all intended to be a good approximation of the "Lateral Screens" section of the material design spec.

The fab.dart example is included as stop gap until the gallery and other demos have been updated.