Previously, if a StatefulWidget was marked dirty, then removed from the
build, then reinserted using the exact same widget under a widget under
a LayoutBuilder, it wouldn't rebuild.

This fixes that.

It also introduces an assert that's supposed to catch SizeObserver-like
behaviour. Rather than make this patch even bigger, I papered over two
pre-existing bugs which this assert uncovered (and fixed the other
problems it found):

   https://github.com/flutter/flutter/issues/5751
   https://github.com/flutter/flutter/issues/5749

We should fix those before 1.0 though.

This eliminates all the issues around the horizontal page transition. I also improved the back gesture animation, and added fling support.

BUG=https://github.com/flutter/flutter/issues/5678
BUG=https://github.com/flutter/flutter/issues/5622

Fixed a bug where the size of the AnimatedCrossFade would always
start from the size of the first child, irrespective of the
initial crossFadeState argument.

Added a widget that cross fades two children while animating the
size of the parent based on the children's interpolated sizes.

Fixes https://github.com/flutter/flutter/issues/5283

Other changes in this patch:

Rename OffStage to Offstage.
Fixes https://github.com/flutter/flutter/issues/5378

Add a lot of docs.

Some minor punctuation and whitespace fixes.

1: If a route is already dismissed when it's popped, there's no point
trying to animate heroes, because it's going to be gone before the
heroes code can look at it.

2: If a hero animation finishes just as a new one is starting, we
previously blew away the state for the starting one. Now we correctly
segregate the "starting up quest" variables from the "actively ongoing
quest" variables.

Somehow we missed this before.

Fixes https://github.com/flutter/flutter/issues/5615

Fixes https://github.com/flutter/flutter/issues/5588

Doesn't do any of the fancy effects. Just lets the user control the
back transition by sliding from the left, like a drawer. Hero
transitions are disabled during the gesture.

BUG=https://github.com/flutter/flutter/issues/4817

Added a widget that implicitly animates the size of it child.

Currently, we just treat Fuchsia as TargetPlatform.android, but we might
want to do something more sophisticated in the future.

* Removed unnecessary Shrine hero logic

Instead of a PNG, the Flutter gallery widget is now drawn in code.

There's now a FlutterLogoDecoration class that paints the flutter logo
anywhere you can use a Decoration (e.g. AnimatedContainer).

There's now a FlutterLogo class that honors the IconTheme.

The About dialog box API now takes a Widget for the applicationIcon,
instead of an ImageProvider. It uses IconTheme to make the icon the
right size instead of using an Image widget.

Add padding, duration, and curve properties to the DrawerHeader.
Make the child of a DrawerHeader optional.

Clean up UserAccuntsDrawerHeader a bit.

Add some useful properties and methods to EdgeInsets.

Add some debug logic to RenderDecoratedBox to catch unpaired
save/restore calls when possible.

Make GestureDetector fill its parent if it has no children. Fixes
https://github.com/flutter/flutter/issues/5380

Fixes https://github.com/flutter/flutter/issues/5380

Added a simple widget that automatically dispatches a
LayoutChangedNotification when its child changes layout.

Added a flag that instructs Stack how to deal with overflowing
children: they can either be clipped or not.

Changes in this patch:
- iOS now uses a different scrollDrag constant than Android.
   - ScrollConfigurationDelegate now knows about target platforms.
   - ScrollBehaviors now know about target platforms.
   - RawInputLine now has to be told what platform it's targetting.
   - PageableList now has a concept of target platform.
- make debugPrintStack filter its stack.
   - move debugPrintStack to `assertions.dart`.
- add support for limiting the number of frames to debugPrintStack.
- make defaultTargetPlatform default to android in test environments.
- remove OverscrollStyle and MaterialApp's overscrollStyle argument. You
  can now control the overscroll style using Theme.platform.
- the default scroll configuration is now private to avoid people
  relying on the defaultTargetPlatform getter in their subclasses (since
  they really should use Theme.of(context).platform).
- fix some typos I noticed in some tests.
- added a test for flinging scrollables, that checks that the behavior
  differs on the two target platforms.
- made flingFrom and fling in the test API pump the frames.
- added more docs to the test API.
- made the TestAsyncUtils.guard() method report uncaught errors to help
  debug errors when using that API.

...so that you can use hot reload mode to update assets.

After this patch, they ignore pointers.

BUG=https://github.com/flutter/flutter/issues/3938

This patch improves the Post and Shrine transitions by making the AppBar
into a Hero and changing the default MaterialPageTransition. Now the
AppBar transitions smoothly between screens and the
MaterialPageTransition doesn't involve a fade effect.

Also, rejigger the bounds of the image header in Pesto to avoid the
"pop" at the end of the animation by laying out the image header at its
final visual size instead of relying on occlusion to size the image
header.

Fixes #5202
Fixes #5204

Why this matters: If you navigate back to a page with a Scrollable that
has a nonzero scrollOffset, we will restore that scrollOffset. We clamp
the scrollOffset to the contentExtent before the first layout, before
contentExtent is updated to its proper value. Initializing contentExtent
to INFINITY effectively disables the first clamp, until we can get a
valid value from layout. Since the previous scrollOffset was valid, it
seems safe to assume it's still valid.

BUG=https://github.com/flutter/flutter/issues/4883
BUG=https://github.com/flutter/flutter/issues/4797

This also required changing the AnimationController state transition
logic to signal completion of the animation during the tick that
finishes the simulation.

Fixes https://github.com/flutter/flutter/issues/3675

Fixes https://github.com/flutter/flutter/issues/5178

Thanks to @leafpetersen for catching these.