Add support for "extended height" toolbars - ToolBar bottom

Added a Widget-valued bottom ToolBar property. If set the bottom widget appears below the normal toolbar.

Removed the kToolBarHeight hardwiring from Scaffold.

Updated the "swipe away" demo to use an extended height toolbar.

Stocks list doesn't update when stocks are loaded

Previously, we passed widgets up the hierarchy to display them in the overlay,
but that breaks the change propagation logic because those widgets won't get
rebuilt.  Now we pass WidgetBuilders instead, which can be rebuilt when the
overlay rebuilds.

Fixes #1913

Tap and the othes classes in tap.dart are not really related.

Make RRect immutable and replace RRect.setRectXY with a fromRectXY co…

Add tests for flx Bundle.

Also cleaned up the flx code a bit. Replaced custom KeyPair class with
cipher's AsymmetricKeyPair.

Adds FlexJustifyContent.collapse

Replace ShowPress with TapDown

Merge DisposableArenaMember and GestureArenaMember

Move velocity tracker to dart

Turns out there were no uses of GestureArenaMember other than those that
use DisposableArenaMember.

CustomMultiChildLayout delegates to a MultiChildLayoutDelegate. Widget children are specified by their index in the list of children.

Rename sky.yaml to flutter.yaml

Routes shouldn't be interactive when animating out

Somehow we missed this one file.

More flutter more better.

This patch introduces the notion of a ModalRoute that puts up a modal barrier
and makes the route invisible to hit testing when its animating out. This patch
also uses this mechanism in a number of places (including PageRoute). There are
still a few more cases to convert, but that's work for a future patch.

Fixes #1684

Move us to a version of sky_tools that doesn't hang when the tests crash.

The name of the parameter wasn't consistent with IconButton or DrawerItem, etc.

Fixes #1871

We were trying to do a hero animation from a page to itself, which doesn't make
any sense. Now we only render the "to" page offstage if it is different from
the "from" page and if its performance isn't already complete.

This test currently crashes the engine.
Because of a bug in the test harness, it then causes the tests to hang.
This disables the test for now, so that we can get test coverage again.

This patch makes it so drag-and-drop doesn't thrash the overlay list.