* Rename hittable to hitTestable

Apparently hittable is not politically correct.

* Enable tracing on Fuchsia

This requires an updated buildroot.

After this patch, we no longer link with Mojo. We still use some gn definitions
from //mojo to create Dart packages.

We no longer use mojom to transport messages. We still use the Mojo EDK
to spin the event loop, however.

This patch depends on another patch that hasn't landed yet.

This reverts commit 960da66f.

* Wire up to mojo::tracing::* infrastructure.

* Initialize tracing as early as possible.

* Make dependency specific to fuchsia.

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

We don't have a way to print stack traces on Fuchsia yet. This patch
isolates this base dependency in //glue.

The ftl::TaskRunner is still based on a base::MessageLoop, but this patch paves
the way to replace the message loop on Fuchsia.

This patch introduces a //glue library that isolates our dependency on //base.
This will let us clean up our //base dependencies ahead of actually being able
to fully remove them.

Now that tonic doesn't depend on anything in //sky/engine anymore, we
can move the code to a location where its dependencies are clearer.

The sky_snapshot tool now understands the --packages command-line flag.

Progress on https://github.com/flutter/flutter/issues/1351