This patch renames StatelessComponent to StatelessWidget and
StatefulComponent to StatefulWidget.

Fixes #2308

Fixes https://github.com/flutter/flutter/issues/1346

...and friends.

This patch teaches the widget framework how to use Cassowary-based
autolayout. To integrate autolayout with widgets, I had to refactor how
RenderAutoLayout worked a bit. Now RenderAutoLayout follows the same
delegate pattern we use for custom paint and custom layout.

The virtual viewport machinery now handles all of these use cases.
Previous clients of ScrollableWidgetList can use ScrollableLazyList
instead.

This patch sketches out the basic widgets for creating Mozart child
views. We're still missing some of the machinery, so we can't quite
create child views yet, but once we supply the ViewHost and teach the
compositor how to actually display the child scenes, we'll have
something that works.

RawKeyboardListener makes it easier to listen for raw keyboard events.

Fixes #787

This patch restructures how we size the editing region of the Input widget. Now
RenderEditableLine understands that it's a single-line editing widget and sizes
itself correctly.

Previously this code was in the animation layer, which didn't make much
sense because scrolling is a widget concern.

ScrollableList2 uses the same pattern as ScrollableGrid, which requires the
client to allocate widgets for every list item but doesn't inflate them unless
they're actually needed for the view. It improves on the original
ScrollableList by not requiring a rebuild of the whole visible portion of the
list when scrolling. In fact, small scrolls can often be handled entirely by
repainting.

We now support (vertically) scrollable grids with viewporting. If the
scroll doesn't reveal any new rows, we execute the scroll with a repaint
(i.e., no layout). If the scroll reveals a new row, we trigger a layout
to change the set of materialized children in the viewport.

Users of MaterialApp can provide an onLocaleChanged handler that will be
called to asynchronously fetch locale-specific data.  MaterialApp will
then instantiate a LocaleQuery that supplies the locale data to its
descendants.

PageRoute is now MaterialPageRoute.

This also changes the following:

- Now the HeroController is a Navigator observer, rather than a feature
  of HeroPageRoutes, which are gone. This means heroes can work between
  any kind of ModalRoute now.

- ModalPageRoute is moved from modal_barrier.dart to routes.dart.

- It allows routes to opt-out of their modal barrier being a shortcut to
  popping the route.

- Features of PageRoute that aren't Material-specific get promoted to
  ModalRoute features: storage, the subtree key, offstageness...

The AnimatedModalBarrier is still a ModalRoute feature.

Somehow we missed this one file.

This patch introduces the notion of a ModalRoute that puts up a modal barrier
and makes the route invisible to hit testing when its animating out. This patch
also uses this mechanism in a number of places (including PageRoute). There are
still a few more cases to convert, but that's work for a future patch.

Fixes #1684

This widget smoothly resizes as its child changes and lets you provide entrance
and exit transitions for its children.

These are now part of material.dart.

Clients should just use a GestureDetector (or an InkWell) instead.

This class has no clients.

Now SnackBar is an ephemeral route that uses a Placeholder to put itself into
the Scaffold.

Fixes #673

Example:
  import 'package:sky/widgets.dart';
  void main() => runApp(new Center(child:
      new StatisticsOverlay.allEnabled()));

This widget is used in Material and Drawer. We don't currently support
animating towards null, but we can add that in a future patch.

* Moves from vector_math to vector_math_64
* Adds support for Float64List in Dart bindings

Instead of having to manage the popup menu from your app's build
function, you now just call showPopupMenu() with the menu's position and
it takes care of everything for you.

This solves the problem that the popup menu was trying to mutate the
state of the navigator from within its own initState() function.

Also, remove the "route" argument to RouteBase.build() since it equals
"this" by definition...

Also, remove ModalOverlay, and instead put that logic in the navigator.