Also, rename SkyViewClient to RuntimeDelegate. These names are more
sensible.

This patch also cleans up the RuntimeDelegate a bit, for example by
removing support for flushing real-time events, which aren't used.

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.

This would have answered a question we received on flutter-dev.

Fixes https://github.com/flutter/flutter/issues/3517

Previously we would hold the layer tree until the we returned from Dart. Now we
send the layer tree to the rasterizer thread immediately.

Fixes #212

The layout protocol in Mozart changed. Now we need to pipe a scene version
through the system to the compositor.

Adds or augments the documentation of various objects in dart:ui.

Removes the ignored argument to SceneBuilder.

Hide the default constructor on Gradient.

Add a call to the ImageShader constructor.

Reorder some members so that private versions come after public versions
so that docs will come before both.

Also, add some dartdocs for AppLifecycleState values.

And trivial docs changes.

This adds some dartdocs to dart:ui, specifically for Window and friends.

We now respect the "route" field in Intents to load a route other than '/'.
Also, use popRoute rather than events to indicate that the framework has asked
us to go back.

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

We don't use ui.view anymore.

Currently this code isn't completely wired up, but this patch is an incremental
step in that direction.

This patch starts down the process of cleaning up the dart:ui API. In this
approach, we use Dart code to specify the interface instead of IDL. The code in
this patch is far from complete. It's just a sketch to see how this sort of
approach might work.