Replace WidgetError and RenderingError with FlutterError

Add dartdoc for child widgets

This patch adds a consistent, simple description for the common pattern of
having a single child widget.

Docs for AvatarCircle.

[driver] actually start the timer when connecting

Tweak the selected tab indicator animation.

Look for the keyboard in the view services

The engine now provides the keyboard and raw keyboard services via the view
services. This patch updates the framework to look there for the keyboard.
Also, this patch migrates callers to the new MojoServices name for these entry
points.

Fix some icon names

At some point we were overly zealous about s/new/const/ in this file.

docs: Updated descriptions for Animation API docs.

More tweaking of how Icons dartdocs.

Made it faster and less stretchy.

Fix markdown errors with last Icons dartdoc patch

Improve the material design icon dartdocs further

Add yet more dartdocs.

Only require documentation for libraries.

Miscellaneous dartdoc markdown fixes

Widgets App fixes and minor newton cleanup

Add a bool to indicate whether we're in MojoShell

More dart docs for text painter

Add Decoration.lerp and document Decoration

- Make the textStyle optional
- Remove routes
- Make onGenerateRoute required
- Add some dartdocs

Update engine.version

Fix position of the overlay

Blocked on https://github.com/flutter/flutter/pull/2771

Now that the overlay actually respects the position we give it, put it
somewhere where the keyboard won't overlap it.