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

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().

(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.

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

Fixes #1168

Fixes #1170

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

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

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

Fixes #1372

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.

Identify specific parts of a Stock row with a Global Key that can be
regenerated later, and pass that key back to event handlers so they can
use them to do the transition.

You can now set an offset that will be used for the hit testing for
drops. (It doesn't move the feedback by default; use a Transform for
that.) I also made the default feedback not be Opacity 0.5 always.

- Rename EdgeDims constructor to EdgeDims.TRBL().

- Add operator== to Size and Offset so that you can compare Size to
  DebugSize in checked mode.

- Add Size.lerp().

- Add various operators to EdgeDims. (*, /, ~/, %)

- Add EdgeDims.lerp().

- Update style guide. I went there to fix an EdgeDims constructor
  example, and stayed because some recent things came up and I wanted to
  add them before I forgot.

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

It's redundant with card_collection.dart, which does similar things but
much better.

This removes GlobalKey.currentElement in favour of GlobalKey.currentContext.

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

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.

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.

Initial snap offset support for ScrollableWidgetList (and ScrollableList<T>) and ScrollableMixedWidgetList. If a ```toSnapOffset(scrollOffset)``` function is provided, fling Scrolls will coast to the returned value. If ```alignmentOffset``` is specified then fling scrolls conclude when toSnapOffset's value lines up with the Scrollable widget's origin + alignmentOffset. For example if the Scrollable widget's height was 200.0, and alignmentOffset:100.0 was specified, then fling scrolls would end with the value returned by toSnapOffset() lined up with the center of the Scrollable.

This approach to Scrollable snapping assumes that the layout of whatever the Scrollable contains is known at the outset. This is often true however a ScrollableMixedWidgetList may not know its items' sizes until they've been reached by scrolling.

This is a first cut at snapping support. Among the things that remain to be done:
- Scrolling limits trump snapping. Snapping should probably trump scrolling limits.
- Drag scrolls aren't snapped. This may be desirable so perhaps the feature should be controlled with a flag.
- Specifying alignmentOffset as a percentage would probably be more convenient.
- It would be nice if one could wrap items in a SnapOffset value like: ```new SnapOffset(0.5, child: myItem)``` to snap to the center of the item.

Updated the CardCollection example: snapping and fixed size items can be turned on/off with Drawer checkboxes.

Tapping on the menu item didn't animate the checkbox because the menu
takes 300ms to animate entirely away while the checkbox takes 200ms to
animate checked, and since the item with the checkbox was at the bottom,
we were only seeing about 60ms of the entire checkbox animation, which
isn't enough to notice it. So I moved it to the top of the menu.

Tapping on the checkbox didn't animate because nothing caused the menu
to rebuild when the callback was invoked. To trigger a rebuild, I now
call navigator.setState() explicitly, after changing out local state.

To make tapping the checkbox remove the menu, I also explicitly call
navigator.pop() in the code that handles the check. (I still explicitly
change the checkbox to show that that's possible. In principle one could
just treat the checkbox as an inert widget that happens to trigger pop,
and do all the checking/unchecking in the switch statement.)

I also made some minor style tweaks to files I was looking at while
dealing with this issue.