Also a bit of code cleanup.

The key part of this patch is the addition in `_endScroll` to reset
`_simulation`. It seems like this was the one place where it's possible
for us to end the animation but not reset our state. Since we assert
that are state is coherent, we were hitting asserts when a fling
finished and then you interacted with the widget again.

Overscroll indicator for MaterialList

Fixes #617

...and fix a typo in a setState assert message.

Previously, when the content extent changed during a scroll interaction, we'd
stop the current scroll interaction and reset the scroll offset. Now we try to
continue the scroll interaction (e.g., drag, fling, or overscroll) even through
the underlying scroll behavior has changed.

For physics-based scroll interactions, we keep the current position and
velocity and recompute the operative forces. For drag interactions, we keep the
current position and continue to let the user drag the scroll offset.

After this patch, we still disrupt non-physical scroll animations that are
operating outside the new scroll bounds because it's not clear how we can
sensibly modify them to work with the new scroll bounds.

As usual, as I was doing this I ran into some stuff that seemed hard
to document as-is and so I changed it. In this case, in the "http"
library. The new code is more or less equivalent, I think, but the
resulting documentation makes it more obvious that it's wrong...

And fix the zillion issues that uncovered.

This reverts commit f41b3411, reversing
changes made to e33d8d96.

This was a bad check-in due to my mangling uploading a new version of the branch from a different machine.

This reverts https://github.com/flutter/flutter/pull/2639 and will be replaced by https://github.com/flutter/flutter/pull/2640

And fix the zillion issues that uncovered.

Fixes #2324

* Makes repeat default to the lower and upper bounds
* Makes animateTo handle unbounded ranges better
* Improve the name of _TweenSimulation

Fixes #2096
Fixes #2090
Fixes #2091

This concept is now private to AnimationController. All the clients actually
want the AnimationStatus.

Previously we would elide forward and reverse callbacks that canceled each
other out, which broke the expected state machine. Now we synchronously deliver
status callbacks when start an animation.

Fixes #1913

Refactor Dismissable to not need a SizeObserver and to generally tighten
up the code a bit.

Rather than clamping _value on read, we now clamp the value when writing it,
which simplifies reasoning about _value.

We should mutate the status of the AnimationController before calling
the status callbacks so that we're prepared to be re-entered.

Fixes #1911

Move StepTween to tween.dart.

Move _buildIndicator() methods to be next to the build() methods they're
used from, so that reading the code is easier.

Fix typos, add some comments, style nits.

Each layer is supposed to reexport the parts of the previous layer
that are part of its API.

- In painting.dart, export from dart:ui all the Canvas-related APIs
  that make sense to be used at higher levels, e.g. PaintingStyle.

- Delete painting/shadows.dart. It was dead code.

- In rendering/object.dart, export all of painting.dart.

- In widgets/basic.dart, export all of painting.dart and
  animation.dart. Some classes in animation/ are renamed to make this
  less disruptive and confusing to the namespace.

- Split out Stocks back into an import model rather than a part model,
  so that it's easier to manage its dependencies on a per-file basis.

- Move Ticker to scheduler library.

- Remove as many redundant imports as possible now.

- Some minor nit picking cleanup in various files.

After the refactoring of the animation library, we were missing some
dartdoc.

When opening the keyboard or focusing an input widget, we should scroll the
widget into view so that the user can see what they're typing.

Previously this code was in the animation layer, which didn't make much
sense because scrolling is a widget concern.

These should be the last references to the old "performance" name.

Some of the Animation classes that we converted from performances use
the term "master" to refer to the animation upon which they're based.
This patch changes them to use the term "parent", which is consistent
with the rest of the animation classes.

Now that we've decided that Animation<double> isn't confined to the interval
0.0 to 1.0, we can expand AnimationController to cover the use cases that used
to require SimulationStepper.

This patch merges SimulationStepper into AnimationController and ports the one
stand-alone client of simulation stepper over to using AnimationController.

ClampedSimulation makes more sense at the newton layer because it's dealing
entirely with newton concepts.

This patch removes Performance and AnimationValue now that we've ported the
framework over to AnimationController and Tween. This patch also cleans up the
names of the AnimationController classes now that they don't have to avoid
conflicts with the old animation API. Specifically, I've made the following
renames:

 * Animated -> Animation
 * Evaluatable -> Animatable
 * PerformanceStatus -> AnimationStatus

This patch is just renames and moving code around. There aren't any changes in
behavior.

There should be no more uses of Performance or AnimatedValue in the framework
or the examples.

After this patch, there aren't any direct uses of Performance in material.dart.

This patch moves Navigator and related code over to using
AnimationController.

Also, clean up the class hierarchy for AnimationController now that
we've renamed progress to value. That means everything in the hierarchy
now has a value, include Watchable. This patch renames Watchable to
Animated<T>, which lets us use that type almost everywhere.

I've added some ducktape to modal bottom sheets to avoid having to
refactor all of Navigator to use AnimationController. I'll remove the
ducktape in the next patch.