This change disentangles the build for the snapshotter from the main engine
build. Also, we now have all the Dart files for dart:ui in the same directory.

We now just call stat directly to test whether a file exists.

We were creating the task runner adaptor before the underlying base::Thread had
a TaskRunner to adapt. Now we wait until after we start the thread.

This patch hides the base::SingleThreadedTaskRunner, which pulls a big
switchover to ftl::WeakPtr and std::function.

Add an association between FlutterViews and Dart Isolates to the PlatformView service protocol support (#2886)

This patch centralizes the asset bundle processing in //flutter/assets and
removes the //base dependency from the code.

* Update to mojo ba13534f2b2af27a1d73e176f7406dbab25f8e14

* Fix build for updated mojo

* Fix android build. Undo some of the mojo roll.

* Fix iOS build.

This required us to fix a backwards dependency from flutter/lib/ui to
sky/shell.

We can use the one from //lib/tonic instead.

This removes most (all?) of our dependency on base::RefCountedThreadSafe.