Fixes #11855

backgroundColor -> color
backgroundImage -> image
BackgroundImage -> DecorationImage

Aggressively apply the const lint.

This ensures that both timer/microtask queues are cleared out rather
than assuming that all work is completed in a specified amount of time.

Rename all the "2" classes related to scrolling to remove the "2". Now that the
old scrolling code is gone, we don't need to use the suffix.

Also fixes a bug when trying to update the offset of a viewport in a layout
builder.

Fixes #8054

(and minor doc changes)

This will in the future allow Scrollable to not track the Simulation itself.

This issue got fixed by a recent check-in, but I had a test specifically
for this incarnation so we should probably keep it just in case.

Closes https://github.com/flutter/flutter/issues/5630