Moving the microtask queue into tonic solves three problems:
1) Removes three levels of indirection when invoking microtask
callbacks.
2) Removes the sky/engine/dom directory entirely.
3) Removes the last client of the (inefficient) DartValue class.
This complex machinery made sense for Blink, which has hundreds of
developers. In our case, we're just moving around two bools. We can do
that in a much simpler way.
This updates to mojo commit d259eb58aa59 and limits the roll script to
only pull in the parts of //mojo that are currently being used. More
stuff will be dropped in the future.
