Make the animator host an implicit fallback VSync provider in case platforms cant provide their own. (#2953)

Now lib/ui depends on sky/engine rather than the reverse.

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

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

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.

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

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

Add collection of PlatformViews to the Shell and allow them to be queried over the service protocol (#2861)

This patch removes almost all //base dependency of //flow. The only dependency
left is on tracing.

Remove dependencies on ui, third_party/mesa and mojo/convertors. Provide simple alternatives. (#2783)

Updating base turns out to be more complicated than I thought.

This reverts commit daa822aa.
This reverts commit c38428c7.
This reverts commit 4b73a8a0.

Update base, gn, and their clients.

This patch disentangles dart:jni from //sky/engine and moves it into
//flutter/lib/jni. Eventually dart:ui will move into a peer directory.

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.

This enables the specification of a local engine (via `--local-engine`)
for flutter tools to use.

We no longer provide the main function for Flutter apps on iOS. Instead, we
just provide Flutter.framework, and the end-developer provides main.

Also, remove unused build rules for building Runner.app. We don't build that
target anymore.

This patch switches us to building Runner.app on the end-developer's machine,
which will eventually let them customize their app.

Fixes https://github.com/flutter/flutter/issues/3256

After this patch, main_ios.mm now consumes Flutter.framework via public APIs.
This patch prepares us to move Flutter on iOS over to consuming
Flutter.framework as a binary artifact.

The flutter tool is now using the observatory's HTTP-based interface

These build definitions no longer work with the current version of FNL. Once
FNL is ready, we'll add new build definitions that work for it again.

All the headers for Flutter.framework are now in framework/Headers and all the
source files for Flutter.framework are now in framework/Source. Previously it
was unclear that FlutterAppDelegate and main_ios.mm weren't part of
Flutter.framework. (They still build as part of the framework, but that's
something I'll fix in a later patch.)

Report Flutter widgets to iOS accessibility API

This patch maps glfw's keyboard input to the raw keyboard Mojo interface.

This patch cleans up the public Java interface to the Flutter engine. The code
that we intend to be re-usable is now in the io.flutter.view package. The two
public classes are:

 * FlutterMain, which controls initialization of the system, and
 * FlutterView, which is an Android view that hosts a Flutter app.

We'll need to interate on these interface over time, but at least these names
are more reasonable.

Add an HTTP server for exporting Flutter debug info and a request han…

This flag makes it easier to enable the glfw configuration.

Also, fix an unrelated build error in loader.cc.