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.

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

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

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.

This patch paves the way to untangling a large amount of code from the engine.

Also, rename TextStyle#lineHeight to height.

We used to share memory between Dart strings and WTF::String objects by
way of the Dart externalized strings. That used to be important when the
DOM shared many strings between C++ and Dart. However, now that we don't
retain strings in C++ much anymore, we don't need this complexity.

This patch removes DartStringCache and the integration. It also unwinds
several cases where we were converting back and forth between
WTF::String and std::string for no reason. Now we use std::string more
consistently.

For the case of ParagraphBuilder::addText, we now take a raw const
char*, which more closely matches the API the DartVM exposes. That means
we do a single copy out of the VM and into the render tree at that
point.

Change line-height to use percentages rather than pixels.

This patch also lets us remove FontFamilyNames, which is one of two
remaining clients of jinja2.

Instead, use our new template magic.

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.

This patch start down the road of implementing text layout and painting without
the DOM. We can construct a basic paragraph consisting of a single run of text
and we can get through layout without crashing.

Rather than using the DOM to upload text and styling information into the
engine, this patch begins sketching a more direct API that bypasses the DOM and
CSS. Currently, this API doesn't do anything, but it's a first step.

The approach is to have a ParagraphBuilder object that can record a tree of
style interior nodes and text leaves. The build() function then applies
container-level styling information (such as TextAlign) and returns a Paragraph
object that can undergo layout and paint.

The inputs to the builder process are immutable style objects constructed from
primitive values. These primitives are currently carbon-copies of the primitive
we use in the framework today. After this patch lands, I'll convert the frame
to re-expose these values instead of re-defining them.