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.

The only client wants a zero-based duration.

This patch prepares us to switch to using integers when handing off the
animation time from the engine to the framework.

Force AnimatedVariables to hit begin on 0.0

We already forced hitting end on 1.0.

Fixes #1358

After this patch, InkWell is driven by gesture recognizers, which lets us
cleanly cancel splashes when the user actually scrolls.

I've also refactored all the clients of InkWell to use InkWell to detect
gestures instead of wrapping InkWell in a GestureDetector.

Fixes #1271

Turns out .. binds tigher than ??= according to
https://www.dartlang.org/docs/dart-up-and-running/ch02.html#operators, which
means we were only updating the callbacks when we first created the
recognizers. Now we update them unconditionally.

 * Rename GestureTapListener (and friends) To GestureTapCallback to match the
   other gesture callbacks.
 * Replace "ensureFoo" pattern with ??= operator.

Fixes #1372

Regression test for #1215

Assert that App(routes) is not null.

The ShaderMask widget enables rendering its child with an alpha channel defined by a Shader. For example if the Shader was a linear gradient in alpha then the component behind the ShaderMask's child would appear wherever the gradient's alpha value was not fully opaque.

The card_collection.dart example demonstrates this. Select the "Let the sun shine" checkbox in the app's drawer.

Add AnimatedContainer

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.

Detecting just Pan events causes an exception in GestureDetector

Fixes #1320

This is probably a sign that you're using fn2 still.