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.

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.