Adds support for applying forces and impulses to sprite physics

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.

Now SnackBar is an ephemeral route that uses a Placeholder to put itself into
the Scaffold.

Fixes #673

Focus.at() and company should be on Focus, not FocusState.

_notifyDescendants() was using the wrong runtimeType.

Let InheritedWidget update the descendants during build.

When you setState() during build, assert that you're not
markNeedsBuild()ing someone who isn't a descendant.

Typo in Widget.toString().

Handle a route being dismissed before being popped

Add more debugging information to Widgets.

(These are changes cherry-picked from in-flight branches since they are
more independent and could be helpful even without those changes.)

- Change RouteBuilder's signature to take a single argument in which the
  other fields are placed, so that we can keep iterating on those
  arguments without having to break compatibility each time. Also, this
  makes defining route builders much simpler (only one argument to
  ignore rather than a variable number).

- Expose the next performance to RouteBuilders, since sometimes the
  route itself might not be where it's used.

- Allow BuildContext to be used to walk children, just like it can for
  ancestors

- Allow BuildContext to be used to get the Widget of the current
  BuildContext

- Allow StatefulComponentElement to be referenced with a type
  specialisation so that you don't have to cast when you know what the
  type you're dealing with actually is.

Handle removal of sprite physics bodies during the physics simulation