Fixes #2662

Update FlutterPlugin.groovy

Provide more documentation for MojoShell

As usual, as I was doing this I ran into some stuff that seemed hard
to document as-is and so I changed it. In this case, in the "http"
library. The new code is more or less equivalent, I think, but the
resulting documentation makes it more obvious that it's wrong...

Refactor cassowary so it uses imports rather than parts.

Improve flutter create help text.

Also misc cleanup:
 - reorder members to be more consistent and fit the style guide
 - remove use of _Pair
 - made Variable.applyUpdate and Variable.owner public
 - added docs to Priority, tweaked the code a bit
 - added some docs to Result
 - removed the internal-error Result (replaced with asserts)
 - removed unused Results
 - made Result const
 - merged some files together since they had used privates a lot

I'm sorry this is completely unreviewable. I did the move from `lib/*`
to `lib/src/*` first, then did the `part`-to-`import` change, and then
found out how many of the files involved privates, which I wasn't
expecting. I can redo this as multiple commits if that would make it
easier to review.

Track metrics for RepaintBoundary.

Fixes https://github.com/flutter/flutter/issues/475

Update the groovy build example.

Refactor CheckedModeBanner into something more reusable.

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