* Move dart:mojo.internal to //flutter/lib/mojo

To be parallel with the other built-in libraries.

* Move DartRuntimeHooks into //flutter/lib/ui

This code manipulates natives.dart, which is part of //flutter/lib/ui.

This pulled a refactoring of how we keep track of the primary threads.

All that's left is two task runners, which should be statics somewhere.

We only need this target for preparing the Dart packages for distribution now.

These aren't needed anymore.

We can't quite build it yet because libwtf.a depends on //third_party/icu,
which we don't have yet, but this patch gets us closer.

This patch removes the //flutter/sky/engine dependency from //flutter/lib/ui,
which lets us build the bulk of the dart:ui library without needing to build
//flutter/sky/engine.

There are now three ways to start Dart applications:

1) From a precompiled snapshot
2) From a snapshot
3) From source

When loading from source, we no longer use the complex data pipe /
dependency catcher mechanism. Instead, we just load the source
synchronously from the file system. This is appropriate because running
from source is used only during development and not during production.

The previous design was built assuming we would want to run from source
over the network, which meant we needed to be able to load source
asynchronously over a high latency channel.

These documents either got moved to the web site (e.g., gestures and
style-guide), became reality (e.g., accessibility and pointer), are outdated
(e.g., events) or are captured in bugs (e.g., keyboard and linkability).

Doesn't quite work yet, but close. Fuchsia is on a newer Dart and will need a
couple patches before this will fully work.

We should eventually rename it to something more sensible, but that's a more
disruptive change. This patch just moves the source code to somewhere easier to
find.

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.