_ModalScope has local copies of route state

Now we just use the state from the Route.

Fixes #744

Make BoxDecoration replaceable.

Factor out a reusable interface called Decoration from BoxDecoration.

Make all the consumers of BoxDecoration and the erstwhile BoxPainter
into consumers of Decoration.

Make a BoxPainter be something you get from a Decoration, rather than
something to which you pass a BoxDecoration.

Rename Shape to BoxShape now that it's documented specifically as
applying to boxes.

Move EdgeDims to its own file.

Move FractionalOffset up so that it's with the other helper classes in
its file rather than alone at the end.

Minor change to RenderClipOval's hit testing to avoid taking an
unnecessary square root.

Rename BoxDecorationPosition to DecorationPosition since
RenderDecoratedBox now takes any Decoration.

Implement hit testing for rounded rects.

Rename AnimatedBoxDecorationValue to AnimatedDecorationValue, and make
it support lerping across any Decoration (by deferring to the objects
involved).

Add a LocaleQuery widget that can be used to fetch locale-specific data

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.

Material gallery crashes when you press the drop-down button

Now use use the route's getPosition function to position the drop-down menu.
Also, fix a number of other related bugs that blocked the dropdown button from
working correctly. The dropdown menu still has the following issues:

1) In the exit animation, the background of the menu disappears too quickly
   because of incorrect paint bounds computations in the layer tree.
2) The drop down menu isn't positioned correctly after the device rotates.
   We'll need to address this issue in a separate patch.

Fixes #630

didChangeNext()

Replace didPushNext() and didReplaceNext() with didChangeNext(), and
call it in more cases, so that a route can easily track the next route.

Use this to make TransitionRoute properly track its next route so that
you can do next-route-driven animations that work even with removes,
replaces, and other crazy manipulations of the navigator stack.

Fix pointer up/move/cancel listeners

Fallout from the pointer refactor.

Navigator.popUntil

Make debugIsVisible work.

This was already supported in the underlying classes but somehow not
exposed by Navigator itself.

AlwaysDismissedPerformance

Debugging aids for overlays, performances

Turns out it was always returning true (visible).

The same as AlwaysCompletePerformance but the exact opposite.

Don't do performance testing in checked mode

The intial route shouldn't run its entrance animation

This makes startup 1500ms instead of 1700ms.

@chinmaygarde @hixie

Instead, it should enter the world fully formed.

Add some dartdoc to basic widgets

Got the check the wrong way in the last patch.

TBR @abarth

Ignore hover events.

These docs are largely duplicated from the cooresponding rendering classes. I'm
not sure how we can avoid repeating ourselves.

Fixes issue with physics debug drawing in sprites

Expose markNeedsBuild on MimicOverlayEntry

We expose this function on OverlayEntry to let you drive animations in the
overlay. The same use case applies to MimicOverlayEntry.

Export the new pointer event classes.

Parent data fixes

Add some missing functionality to ParentData subclasses.

Fixes #474

Update Flutter API docs continuously