Now we only use it for things related to ModalRoute and ModalBarrier.

(This is easy now that ephemeral routes are gone, so there's no other
kind of route to distinguish against.)

- Removed the concept of ephemeral routes.
- Renamed the two _MenuRoutes to _PopupMenuRoute and _DropDownRoute.
- Added type arguments in various places:
  - DropDownMenu
  - _DropDownRoute
  - _ModalBottomSheetRoute
  - PopupMenuItem
  - _PopupMenu
  - _PopupMenuRoute
- Made _ModalBottomSheetRoute, the two ex _MenuRoutes, and _DialogRoute
  all inherit from ModalRoute, via PopupRoute.
- Change "Dropdown" and "DropDown" to "DropDown" consistently.
- Made MaterialPageRoute inherit from PageRoute.
- Made ModalBarrier not create a box if it's always transparent.
- Exposed the Futures on TransitionRoutes.
- Fixed that menus were no longer dismissable by tapping the modal
  barrier.

These end up not actually being used, currently, because we don't have
generic methods, which you'd need for showDialog() and friends, and we
don't have any way to parameterise a class type at runtime, which you'd
need for MaterialApp routes, but it's a step in the right direction.

- `Scaffold.of(context).showBottomSheet(widget);`
- Returns an object with .closed Future and .close() method.
- Uses a StateRoute to handle back button.
- Take the Navigator logic out of the BottomSheet widget.
- Support showing a sheet while an old one is going away.
- Add Navigator.remove().

"showSnackBar()" is now a feature of a Scaffold. To get to a Scaffold
you either use a global key (`scaffoldKey.currentState.showSnackBar(...)`),
or you use `Scaffold.of(context)`.

Snack bars no longer have a route. They are entirely managed by the
Scaffold. Fixes #432.

Snack bars now queue up when you have several of them. Fixes #374.

Snack bars now auto-size themselves around their contents. This is step
one towards implementing multiline snack bars.

Snack bars now self-dismiss after some per-snackbar configurable period.

The self-dismissing pauses while a dialog is up above the snackbar (or
anything that uses ModalRoute). To enable this, there's now a
`ModalRoute.of(context)` API that returns the current ModalRoute, and
you will be rebuilt if you asked for this and the route's "current"
status changes. To implement this, the Navigator now rebuilds
unconditionally any time it pushes or pops a route.

Snack bars now use the curves that Android uses for snack bars.

Snack bar contents now fade in.

I'm planning on adding more .of() functions and so to avoid further
copypasta I'm providing some general utility functions here.

This makes HeroController ignore StateRoutes when deciding where to
animate heroes to and from.

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.

We need to use the performance for the "from" route when going backwards. Also,
fix a bug in the HeroParty where it would call the quest finished callback
multiple times.

Fixes #1958

These types belong at the MaterialApp level.

Fixes #1969

- runApp(new MaterialApp()) was crashing long after the constructor. Now
  it asserts in a more useful location.
- remove the default name for NamedRouteSettings. It was unused anyway.

- Change RouteArguments to pass the route's BuildContext rather than
  the Navigator. This caused the bulk of the examples/ and .../test/
  changes (those are mostly mechanical changes). It also meant I could
  simplify Navigator.of().

- Make initState() actually get called when the State's Element is in
  the tree, so you can use Foo.of() functions there. Added a test for
  this also.

- Provide a RouteWidget so that routes have a position in the Widget
  tree. The bulk of the route logic is still in a longer-lived Route
  object for now.

- Make Route.setState() only rebuild the actual route, not the whole
  navigator.

- Provided a Route.of().

- Provided a Route.writeState / Route.readState API that tries to
  identify the clients by their runtimeType, their key, and their
  ancestors keys, up to the nearest ancestor with a GlobalKey.

- Made scrollables hook into this API to track state. Added a test to
  make sure this works.

- Fix the debug output of GestureDetector and the hashCode of
  MixedViewport.

- Fixed ScrollableWidgetListState<T> to handle infinite lists.

Now you don't need to pass the navigator around everywhere.

This grid makes it significantly easier to track down errors when you're
looking at coordinates in a render tree dump and want to see if they're
what you expect or if they're wildly away from the right location.

Assert at build time that PageRoute route builders do not return null
widget trees.

Also very minor new code comments and code reorg to help make the heroes
patch easier to review.

(These are changes that are unrelated to Heroes but that were part of
the Heroes patch.)

The style we use for callbacks in widgets is "onFoo". These classes were using
an order naming convention and just called their callbacks "callback".

...once the animation is done.

These are now part of material.dart.

It was trying to grab the route's performance before it was created. Now we
create the performance eagerly again.

Previously, RenderObjectElements didn't support being marked dirty. This
is fine, except for MixedViewport and HomogeneousViewport, which have
builder functions to which they hand themselves as a BuildContext. If
those builder functions call, e.g., Theme.of(), then when the theme
changes, the Inherited logic tries to tell the RenderObjectElement
object that its dependencies changed and that doesn't go down well.

This patch fixes this by making RenderObjectElement a BuildableElement,
and making MixedViewport and HomogeneousViewport hook into that to
rebuild themselves appropriately.

Also, this was only found at all because ThemeData didn't implement
operator==, so we were aggressively marking the entire tree dirty all
the time. That's fixed here too.

Also, I changed card_collection.dart to have more features to make this
easier to test. This found bugs #1524, #1522, #1528, #1529, #1530, #1531.

This patch prepares us to create routes that control their performances more
closely.

The margins make using showMenu's menuPosition argument difficult and they're not really needed.

I also made a few small gratuitous changes in navigator.dart.

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

(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.

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

Fixes #1170

This is step 1 in making it possible to have hero transitions between
routes. To make it possible for a route to have an "exit" animation when
a new route has been pushed on top of it, we provide the next route's
AnimationPerformance to the build function. It's null if there is no
next route or if the next route has no performance.

...for consistency with the other Route subclasses:
```
  dialog.dart:      class DialogRoute extends Route
  drag_target.dart: class DragRoute extends Route
  navigator.dart:   class PageRoute extends Route
  navigator.dart:   class StateRoute extends Route
  popup_menu.dart:  class MenuRoute extends Route
```

All the use cases for EventDisposition are now addressed by the gesture
detection system.

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.

- Fix AnimationTiming to have defaults for 'interval' and 'curve' since
  that seems to be how we use it.

- Merge RouteBase.build and RouteBase.buildTransition

- Get rid of HistoryEntry, since it added nothing

- Broke out RouteBase.createPerformance() so subclasses can change what
  is created.

- Build the routes backwards so that we more efficiently avoid building
  hidden routes.

- Introduce an explicit way (!hasContent) for RouteState to avoid
  building, rather than the implicit "build returns null" we had before.

This fixes #1103.

Move the animation libraries into src/animation and change importers to use
package:sky/animation.dart. Also, move scheduler.dart into the animation
library so that the animation library can be self-contained.