SnackBar throws exception on creation

Added horizontal and vertical alignment properties to Stack so that the origin of non-positioned children can be specified. Currently all of the non-positioned children just end up with their top-left at 0,0. Now, for example, you can center the children by specifying verticalAlignment: 0.5, horizontalAlignment: 0.5.

Added IndexedStack which only paints the stack child specified by the index property. Since it's a Stack, it's as big as the biggest non-positioned child. This component will be essential for building mobile drop down menus.

Added a (likely temporary) example that demonstrates IndexedStack.

It was trying to grab the route's performance before it was created. Now we
create the performance eagerly again.

The Drawer should animate out when popped off the navigator

FlatButton highlights but doesn't tap around edge

Adds support for applying forces and impulses to sprite physics

Now we trigger the highlight from the InkWell so that it matches the tap.

Fixes #1525

Now with more clear ownership over the Performance.

Previously, RenderObjectElements didn't support being marked dirty. This
is fine, except for MixedViewport and HomogeneousViewport, which have
builder functions to which they hand themselves as a BuildContext. If
those builder functions call, e.g., Theme.of(), then when the theme
changes, the Inherited logic tries to tell the RenderObjectElement
object that its dependencies changed and that doesn't go down well.

This patch fixes this by making RenderObjectElement a BuildableElement,
and making MixedViewport and HomogeneousViewport hook into that to
rebuild themselves appropriately.

Also, this was only found at all because ThemeData didn't implement
operator==, so we were aggressively marking the entire tree dirty all
the time. That's fixed here too.

Also, I changed card_collection.dart to have more features to make this
easier to test. This found bugs #1524, #1522, #1528, #1529, #1530, #1531.

Separate Route and PerformanceRoute

This patch prepares us to create routes that control their performances more
closely.

Make checkbox widget to fit more closely to  material design standard. Make changes to remove animation artefacts.

Remove PopupMenu margin

The margins make using showMenu's menuPosition argument difficult and they're not really needed.

I also made a few small gratuitous changes in navigator.dart.

The margins make using showMenu's menuPosition argument difficult and they're not really needed.

I also made a few small gratuitous changes in navigator.dart.

NetworkImage occasionally does not grab the image

Improvements to sprite physics

I haven't been able to reproduce this bug consistently, but my theory is that
the ImageDecoder was being garbage collected before it called its completion
callback. This patch prevents that by keeping a reference to the image decoder
while the callback is in flight.

Fixes #801

Simplify Scaffold

ToolBar with a TabBar shouldn't have a shadow

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

Fixes to demo game

PageableList should take initialPage instead of initialScrollOffset

This state object has a public API.

This patch makes the level of the ToolBar configurable. I've also cleaned up
the Tab code slightly.

For some reason, there's still a hairline between the ToolBar and the TabBar.
We might need to rethink how we draw the background a bit here.

Fixes #1454

In the vast majority of cases, folks should be interacting with the Widget
rather than its State.

Fixes #267

Fixes #900

This class has no clients.

Fix Focus

Use Navigator to drive SnackBar

Instead of waiting until build(), we now create the performance for a route in
its constructor.