Scrollable sometimes pushes frames forever

Update engine.version

Fix _LazyWidgetProvider indexing, move Ali Connors

We were setting a negative velocity tolerance, which never triggered because we
assume tolerances are positive numbers. Now we use a positive tolerance and
assert to catch this case in the future.

Fixes #2765

Allow customizing of Switch thumbs

Fixes #2621

Flexible AppBar with a TabBar

Improve error message when ChildView lacks a MediaQuery

refactor; fix apk warning

add support for a FLUTTER_DEV env var in the flutter script

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.