This patch integrates Mimicable with Overlay such that you can tell a Mimicable
to lift up into the overlay and animate towards another widget identified by a
global key.

Rename _updateScrollBehaviour to _updateScrollBehavior

Fix crash when going back in stocks app.

Fixes #675

Heroes with the same tag have to have keys that are unique across the
entire subtree. Since we can now show both stock lists, this means we
have to include the tab in the heroes' keys.

Fixes #668.

Remove animation scheduler.

Merge the two schedulers.

Initialize the track color fields in _RenderSwitch

Add output to "flutter apk" indicating where the APK file was written

Generate an Android manifest in "flutter init"

Remove the assert when opening popup menus.

Some paint bounds are in fact empty, e.g. at the start of a growing
animation if there's also a fade, the initial 0x0 box will also be
opacity:0, and might get a dedicated layer and PaintingContext.

Model ink splashes more physically

Update package:test to the latest version

Makes it easier to hit IconButton

The right fix here is to use a radial reaction, but for now we can just make
the gesture detector including the padding acround the icon in its hit test
region.

Fixes #575

Fix Input padding to match the material spec

Also, handle the case of "dense" input widgets that have less margin.

Fixes #626

Add input#hideText

This feature can be used to implement password fields, where the user doesn't
want the text displayed on screen. Currently all the characters are hidden.
Eventualy we'll want to show the most recent character for some timeout.

Fixes #636

NavigatorTransaction

To make it easier to avoid pushing twice in one frame, provide a
transaction mechanism for the navigator.

Add a default onEnter/onExit for EnterExitTransition

PopupMenu shouldn't use CustomPaint

We now default to the identity transition.

Bottom sheets shouldn't dismiss when flung up

Align is now powerful enough to do all the work we need to do here.

New methods for manipulating Navigators.

* NavigatorState.replaceRouteBefore()
* NavigatorState.removeRouteBefore()
* NavigatorState.popUntil()

Add a "replace()" API for the navigator.

This lets you dynamically replace one route with another.