GridPaper updates

Fixed a tabs scrolling typo

Change how GridPaper works so that it's easier to plug in anywhere.
It now takes a child that it will paint the grid over.
It defaults to semi-transparent grid-paper-blue.
The line spacing is also more configurable now.

Optimize repainting in Scaffold

Fix tab fling scrolling when the last tab is selected

Previously, we triggered a layout (and hence a repaint) when sliding the
draw because we gave the draw loose constraints. The drawer uses an
Align to move itself to the proper side of the screen, so it can have
tight constraints, which makes it a layout boundary.

Also, don't trigger a layout just because the Scaffold rebuilds. There
isn't any state in the scaffold custom layout, so it doesn't need to
repaint just because we created a new instance of the delegate.

Finally, add the debugging infrastructure I used to find these issues.

generate docs for sprites

Reimplement TabBarSelectionState.of() using new methods

Tabs don't display items other than 0 and 1

The TabBarView wants to remap page indices during tab transitions, which
I broke when I refactored TabBarView.

Fixes #1119

Limit the size of the image cache

Fixed #1009

Handle trying to upgrade Flutter with no upstream

Move default back behavior to FlutterWidgetBinding

Fixes #1084

Remove BlockDirection in favor of ScrollDirection

Fix Scrollable.of()

`flutter start` crashes when port 8181 is in use

We'll probably renaming ScrollDirection to Axis in a future patch.

Fixes #151

Catch the error and log a warning.

Fixes #1050

TabBarView material gallery demo

Previously MaterialApp was responsible for ending the activity when the
back stack was empty. However, this behavior is more general than
material. This patch moves the behavior to FlutterWidgetBinding, which
has a global view of all the binding observers.

Fixes #1086

Make ancestorStateOfType() and ancestorRenderObjectOfType() support
subclassing.

Fixes https://github.com/flutter/flutter/issues/1087

Rationalise all our exception catching and reporting code

- make them all have the same style
- make them all include the stack trace last
- make them all stop printing if their callback is set (if
  appropriate, they don't yet all have callbacks)

Remove unreferenced getter "debugDoesLayoutWithCallback".

I think I wanted to use this at some point but ended up going a
different direction. Instead, object.dart has a private field that
serves a similar purpose.

AnimatedContainer has an issue with one config value change and not another

Revert "Make `flutter listen` exit nicely when missing dependencies"

Make `flutter listen` exit nicely when missing dependencies

If we're already at the target value, we fail to configure the variable. If
another variable animates, we re-animate the other variable.

Fixes #958

Previously it died with a confusing exception.

@chinmaygarde

Enable more asserts when building with virtual viewports