Rather than using mojom to encode pointer data, we now encode and decode it
manually. A future patch will remove the mojom codepath once the framework is
updated.

This patch makes three API changes:

 * The dart:ui library now always communicates in physical pixels. The
   framework is responsible for converting to whatever logical coordinate
   system it wishes to use.
 * The textBaselien property is now on TextStyle rather than ParagraphStyle,
   which will let us choose which baseline to use on a per-span basis rather
   than on a per-paragraph basis.
 * This patch also removes the old MojoServices function endpoints.

Fixes https://github.com/flutter/flutter/issues/3779
Fixes https://github.com/flutter/flutter/issues/1360

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.

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.

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.