This patch combines embedder.dart and shell.dart into one thing. We should now
handle a bunch of error cases better.
* embedder.connectToApplication has moved to shell.connectToApplication,
matching the rest of the mojo universe.
* embedder.connecttoService has moved to shell.connnectToService (and merged
with shell.requestService).
* shell.requestService is now shell.connectToService, matching the rest of
the mojo universe.
* serviceRegistry has moved from embedder.serviceRegistry to a top-level
getter.
Fixes#1803
Also, introduce Colors and Typography to hold the material colors and the
typography declarations. Previously we expected clients of these libraries to
import them into a namespace, but that doesn't play nice with re-exporting them
from material.dart.
This still leaves Flex and FlexDirection available. At some point once
people have transitioned to Row/Column we should rename Flex to _Flex
and stop reexporting FlexDirection from basic.dart.
Similar to widgets.dart, rendering.dart exports the entire rendering layer.
Also, update the examples to use rendering.dart and widgets.dart. Also clean up
some exports so that the examples have more sensible imports.
Sadly, box.dart has grown much longer than 1000 lines. This patch splits it up
into several files based on the class hierarchy. Fortunately, many of these
classes are loosely coupled to each other.
