I haven't been able to reproduce this bug consistently, but my theory is that
the ImageDecoder was being garbage collected before it called its completion
callback. This patch prevents that by keeping a reference to the image decoder
while the callback is in flight.

Fixes #801

Simplify Scaffold

ToolBar with a TabBar shouldn't have a shadow

Rather than using a custom render object, we can just use a Stack.

Fixes to demo game

PageableList should take initialPage instead of initialScrollOffset

This state object has a public API.

This patch makes the level of the ToolBar configurable. I've also cleaned up
the Tab code slightly.

For some reason, there's still a hairline between the ToolBar and the TabBar.
We might need to rethink how we draw the background a bit here.

Fixes #1454

In the vast majority of cases, folks should be interacting with the Widget
rather than its State.

Fixes #267

Fixes #900

This class has no clients.

Fix Focus

Use Navigator to drive SnackBar

Instead of waiting until build(), we now create the performance for a route in
its constructor.

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

Fixes #673

Focus.at() and company should be on Focus, not FocusState.

_notifyDescendants() was using the wrong runtimeType.

Let InheritedWidget update the descendants during build.

When you setState() during build, assert that you're not
markNeedsBuild()ing someone who isn't a descendant.

Typo in Widget.toString().

Handle a route being dismissed before being popped

Add more debugging information to Widgets.

(These are changes cherry-picked from in-flight branches since they are
more independent and could be helpful even without those changes.)

- Change RouteBuilder's signature to take a single argument in which the
  other fields are placed, so that we can keep iterating on those
  arguments without having to break compatibility each time. Also, this
  makes defining route builders much simpler (only one argument to
  ignore rather than a variable number).

- Expose the next performance to RouteBuilders, since sometimes the
  route itself might not be where it's used.

- Allow BuildContext to be used to walk children, just like it can for
  ancestors

- Allow BuildContext to be used to get the Widget of the current
  BuildContext

- Allow StatefulComponentElement to be referenced with a type
  specialisation so that you don't have to cast when you know what the
  type you're dealing with actually is.

Handle removal of sprite physics bodies during the physics simulation

Ensure that if a route's performance is dismissed before the route is
popped, that we pop the route.

Also, fix comment mentioning syncConstructorArguments.

Give Interval a Curve to apply between start and end

This patch simplifies AnimationTiming and all the AnimatedValue base classes.
Also, make PopupMenu a stateless component because it has no state.

Fixes #1168

Prevents sprite update methods to be called before the sprite box has…

Improves names of animation classes

Fixes #1170

Allow displaying compositor statistics via a widget

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

Convert Drawer to using navigator

Remove AnimatedSimulation

This patch converts drawer to using the "openDialog" pattern for managing its
state. Currently, the drawer entrance and exit animation aren't integrated with
the navigator's animation system because the drawer's animations can be stopped
and reversed, which the navigator can't yet understand. That means dismissing
the drawer via the system back button causes the drawer to be removed
instanteously.

Fixes #715
Fixes #1187

This patch folds the functionality from AnimatedSimulation into Timeline.

Simplfy resetting board and mine generation.

Rather than having two objects driving scrolling animations, we now have one
object, a Timeline, drive both scrollTo and fling animations. Using Timeline
instead of AnimatedSimulation paves the way to removing AnimatedSimulation
(which is now used only inside the animation library).

Finally, this patch also simplifies (and makes private) _TweenSimulation by
using AnimatedValue to do the math.