Fixes https://github.com/flutter/flutter/issues/8809

We no longer offer mojo handles via this mechanism.

Previously the Dart entry points weren't wired up.

This transport uses the Dart and JNI APIs directly instead of indirecting
through Mojo.

We can now use the real thing on Fuchsia.

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.

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 removes most (all?) of our dependency on base::RefCountedThreadSafe.

* Adopt //lib/tonic/logging

* Adopt //lib/tonic/scopes

* Adopt //lib/tonic/parsers

Also, switch over to //lib/tonic's typed data classes.

This patch moves the remainder of painting and compositing into their
new home (and updates their style to match Google C++ Style).

This should make almost every Canvas call significantly faster.

These parts don't depend on engine anymore. Also reformat them to match Google
C++ style.

Now that tonic doesn't depend on anything in //sky/engine anymore, we
can move the code to a location where its dependencies are clearer.

Particularly, add support for blur filters to implement backdrop blur effects.

We used to use DrawLooper for drawing shadows, but we don't use it anymore.

This patch merges these functions into dart:ui, which is simpler.

Fixes #83

This also extends DartClassLibrary to support multiple DartClassProviders
for different libraries

This patch removes the last IDL file from the build and unhooks the bulk
of the IDL compiler from the build system. A later patch will actually
delete the IDL compiler.

After this patch, Path is the only interface using IDL.

- Gradient
- ImageShader

Instead, use our new template magic.

Rather than using IDL code generation, we now use some template magic to
generate this code. No all the code is template-generated yet, but this
patch is a start in that direction.

This feature is redundant with the Timeline in dart:developer. I've already
switched all the clients over to using dart:developer.

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

This patch changes how we lookup native functions from Dart to be more
extensible. This patch paves the way for removing the dart:ui_internals
library.

We don't use the DOM or CSS anymore. Instead, we work directly with the render
tree.

The NewTextPainter is still disabled by default. A future patch will flip the
flag to enable it.

This patch uses a new approach to writing bindings by encoding data into array
buffers. This approach is more efficient than the existing IDL based approach.
If this works out well, we should convert our other performance-sensitive
interfaces to this approach in future patches.

Now this code is just in dart:ui.