This reverts commit 1e9e134d.

* Revert "Initial integration of libtxt with Flutter alongside Blink. (#3771)"

This reverts commit c548c65b.

* Revert "Call Selection.removeSelection if the framework has cleared the selection (#3782)"

This reverts commit e5b79ba6.

* Revert "Update switches to use StringView. (#3781)"

This reverts commit 07d4357b.

See https://fuchsia-review.googlesource.com/#/c/31931/.

* 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.

Add a —disable-observatory flag to explicitly disable observatory even in non-product modes. (#3012)

Add “—endless-trace-buffer” to switch to an endless buffer from a ring buffer while tracing. (#2966)

* Add config properties to specify snapshot blob file names.

This adds the ability of the shell to override the default
dart aot snapshot blob file names, and it wires up Android's
FlutterMain to recognize the properties in the app's manifest.

This will be used for flutter applications that build their
binary snapshots into files other than the default ones
that the engine uses.

When developing the engine from Xcode, it is useful to see log output in Xcode's UI. We were redirecting streams away so that the tools could pick read these logs. With the change to the scheme, when running from Xcode, the logs will show up in Xcode's UI. When, running from the tools, that flag will not be present and the tools will work as they always have.

This include build system changes for selecting Dart's precompiler mode
plus a way to locate and load the precompiled snapshot library from an
Android application

SkyShell now supports --packages instead of --package-root.

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.

We've been enabling it in the flutter tool for a while. This patch enables by
default in general.

Now 'flutter start --trace-startup' can trigger this tracing
on Android

@chinmaygarde

Also, remove the --snapshot flag because using FLX files is the path we're
paving.

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

Progress on #400

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.

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.