Ensure that the item under the focal point stays in the same place despite zooming

Port input_test to fn3

Make the fn3 EditableText expand to fill the width of its parent

Port dimissable_test.dart to fn3

Implement working UpdateTask in Dart

Dart code now supports the full flow that the C++ code used to: version check,
download, replace app bundle. Bonus: the Dart code is much easier to follow,
thanks to async/await!

This is part 2 of a 3-part change. The first part added new mojom
interfaces, PathService and UpdateService, to the sky_services package.

Snap scrolling: additional tests, cleanup

Simplify AnimationPerformance

s/sky-dev/flutter-dev/

AnimationPerformance had some logic for supporting multiple variables
that was hardly ever used. ValueAnimation, a subclass, has logic for
handling a single variable. I've removed the logic for handling
variables from AnimationPerformance in favour of most call sites instead
using ValueAnimation.

This argument isn't needed anymore now that State has a getter for context.

Add widgets_next.dart to help folks transition to fn3

Also, fix analyzer warning in fn2.

Dynamic named routes

Make it possible for named routes to be generated on the fly.

To demonstrate this, you can now long-press a stock to open it.

Next steps:

 - transitions between (named) states that follow full material logic,
   e.g. in the case of the stock row to stock page transition, expanding
   the row into a raised sheet of material and expanding it to fit the
   screen, leaving the toolbar in place but cross-fading the old
   contents to the new contents.

 - more information in the stock view.

While I was here I also made Material have an opinion about default text
style, so if you forget to set one, it just uses body1.

Also, fixed bugs introduced recently that made RouteState and MenuRoute
not work properly.

Change OverflowBox API to allow min and max values

Make Stocks demo list rows clickable

Fix a couple of analyzer warnings

Introduce a Draggable class that wraps all the logic of dragging
something and dropping it on a DragTarget.

Update examples/widgets/drag_and_drop.dart accordingly.

Make the performance/transition part of routes optional.

Not having them clickable was making it hard to notice InkSplash bugs.
Also, this paves the way to having a stock page.

Make hit testing work in horizontal scrolling list

fn3: toString() and toStringDeep() debugging aids

Require giving a GlobalKey to UniqueComponent

Make the checkbox in the stocks popup menu work.

Previously OverflowBox was only useful to set a tight constraint on the
child. Now it can be used to set any constraint, it just overrides any
constraint from the parent that is not set to null on the
RenderOverflowBox object when giving the constraints to the child.

Previously, hitTestChildren in RenderBlockViewport just didn't support
horizontal scrolling, due to an oversight.

Land snap_scrolling_test.dart

Make the FAB move up when a Snack Bar slides in.

Fix _scrollOffsetIsInBounds analayzer warning