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

The patch teaches Travis how to update our API docs continuously as we land
patches in master.

Repeated background images should be clipped to the container

Fixes #766

Add a completer to MaterialPageRoute

LayoutId should default to using its id as its key

This default seems likely to be benefitial because typically the LayoutIds
coorespond to semantic slots in a bespoke layout.