Add a basic sky_shell.mojo

Make the user specify the path to the flutter repo in Xcode

Currently sky_shell.mojo can't load any Flutter apps or draw to the screen, but
that will come in later patches.

Move platform-specific code for sky/shell into sky/shell/platform

This patch prepares us to add support for Mozart.

The dart_pkg rule for sky_engine now preserves directory structure. Account for the same in SnapshotterInvoke

Fix sky_build.py

The dart_pkg rule for sky_engine now preserves directory structure. Account for the same in SnapshotterInvoke

Move to the service/pointer Mojo interface for input events on Mac

Move to the services/pointer Mojo interface for touch events on iOS

Hardware key support

Add the ability to dispose host objects

Some host objects hold on to a lot of state and take time to tear down. This
patch adds the ability to dispose them eagerly to release resources faster.

Switch to using callback to get observatory data (working)

Rev sky_engine and sky_services

Replace the legacy pointer input event handling with PointerPackets

With this change we no longer need engine/core/events and its IDL bindings.

Remove examples directory

support for POST requests in UrlLoader

Target armv7 when the dart_target_arch is forced to be arm (vs arm64)

Extract jinja template rule from Android specific rules

Move sky_snapshot to the correct place

Improve error handling in sky_snapshoter

Now we print a more sensible error message and exit with an error instead of
appearing to crash.

Fixes https://github.com/flutter/flutter/issues/53