* Namespaces have been updated to reflect the move from //flutter/sky/shell to //flutter/shell.
* shell/BUILD.gn file has been split into smaller GN files for each subcomponent of the shell (common, GPU, diagnostic, testing).
* GN dependencies have been rewritten to stop exposing common shell dependencies as public. Duplicates have also been removed.
* GPU subcomponent has been updated make it more suitable for Vulkan integration.
* The GLFW backend has been resurrected.

It is called each time the embedder initializes a Flutter view controller. The Mac shell (which does not have a view controller), calls it before NSApplicationMain.

Common items like the base::AtExitManager and the platform message loop are stored in `EmbedderState`

If a custom ICU data file path is specified, attempt loading from that instead of loading from the default path.

Allow specification of the FLX bundle (containing no Dart snapshot) outside the iOS/Mac application bundle.

Allows launching of apps without having any Xcodebuild step in the simulator.

This patch prepares us to add support for Mozart.

Now you can use --snapshot to load snapshots from the command line and have
them appear visually on Mac. Also, remove code duplication with Linux by
factoring common testing code into //sky/shell/testing.

We need a way for developers to use release builds in checked mode.

Progress on #400

Also, make stocks robust to failing network connections.

TBR=eseidel@google.com

Review URL: https://codereview.chromium.org/1229743002 .

This does way more than a single CL should do, sorry.
- Fixed a bunch of logging/error reporting during loads.
- Made it possible for --package-root to be absolute.
- Fixed (hypothetically) parts of iOS build to be clang-plugin-compliant.
- Made all tests pass in sky_shell
- Removed a bunch of layout_tests including windows support.

I'll remove all the mojo test harness stuff in a follow-up patch.

R=abarth@chromium.org, abarth@google.com

Review URL: https://codereview.chromium.org/1215953006.

This CL makes it possible to run sky/tests/raw/bogus_import.dart in sky_shell
on Linux. We'd eventually like to scale this up to running all the tests.

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/1218633003.

This CL builds an APK for MineDigger that works offline. We use |sky_packager|
to snapshot the Dart code, which we then extract from the APK during startup.
If we succeed in extracting a snapshot, we load it by default.

TBR=eseidel@chromium.org

Review URL: https://codereview.chromium.org/1211253004.

Unbreak support for snapshots now that we have our offline Dart loader. Also,
wire up support for snapshots in sky_shell on Linux.

TBR=eseidel@chromium.org

Review URL: https://codereview.chromium.org/1212623002.

This CL makes it possible to run Sky apps offline by introducing new
DartLibraryProvider subclasses, specifically one that can load directly from
the file system. A future CL will expand this functionality to work with asset
bundles as well.

Currently, the only platform that uses this functionality is Linux, which can
now load a simple Sky app without even loading a network stack. Making this
work on other platforms is work for future CLs.

TBR=eseidel@chromium.org

Review URL: https://codereview.chromium.org/1203143004.

This CL makes it possible to build sky_shell on Linux. It doesn't do much, but
it doesn't crash on startup. The goal is to evetually be able to test sky_shell
on the bots and to be able to run the test_sky on Mac.

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/1202323002.