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.
Rather than using IDL code generation, we now use some template magic to
generate this code. No all the code is template-generated yet, but this
patch is a start in that direction.
We still use SkPicture as our rasterization backend, but now we're uploading
our layer tree to C++. A future patch will push the layer tree deeper into the
system and we'll eventually switch backends.
