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.

See https://github.com/flutter/flutter/issues/4478

Sometimes it is convenient to construct a color from an opacity rather than an
alpha value.

Fixes https://github.com/flutter/flutter/issues/5205

Apparently Skia would like to remove these enum values.

Fixes https://github.com/flutter/flutter/issues/5912

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

Also, with the comments.

We were off by one on, uh, everything.

* Added drawArc to Canvas.

Added a way to call SkCanvas::drawArc from Dart. This method is
optimized for the arc case and should be faster that drawing paths.

* Removed implementation details from docs.

* Added more details to the docs.

We can now use the real thing on Fuchsia.

After this patch, Flutter apps on Fuchsia can access their asset
bundles. Unlike other platforms, the asset bundles are stored in memory
rather than in the file system.

Also, clean out some unneeded dependencies from engine/platform.

We don't yet have support for text on Fuchsia. This patch add stub
implementations of Paragraph and ParagraphBuilder so that Flutter apps
that use text don't error out. Instead, the text is 0x0 and invisible.

Also, teach MojoServices not to crash if it hasn't been created by the
time Dart tries to use it.

Fixes https://github.com/flutter/flutter/issues/5424

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.

We're now producing a flutter_content_handler binary for Fuchsia that
builds and links.

Now lib/ui depends on sky/engine rather than the reverse.

* Move dart:mojo.internal to //flutter/lib/mojo

To be parallel with the other built-in libraries.

* Move DartRuntimeHooks into //flutter/lib/ui

This code manipulates natives.dart, which is part of //flutter/lib/ui.

This pulled a refactoring of how we keep track of the primary threads.

All that's left is two task runners, which should be statics somewhere.

This patch removes the //flutter/sky/engine dependency from //flutter/lib/ui,
which lets us build the bulk of the dart:ui library without needing to build
//flutter/sky/engine.

Doesn't quite work yet, but close. Fuchsia is on a newer Dart and will need a
couple patches before this will fully work.

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.