Block -> BlockBody
ScrollableBlock -> Block
FixedHeightScrollable -> ScrollableWidgetList
VariableHeightScrollable -> ScrollableMixedWidgetList
BlockViewport -> MixedViewport

1. We would assert if you tried to start an animation from within an
animation status callback. This is a common pattern, so I fixed this
assert (in Ticker._tick).
2. We would assert for animations with duration under a millisecond.
Fixed.

Also removed the workarounds assocated with #1.

We only use them for the transition to dismissed.

Also add intervals and curves to AnimationPerformance, which affect all
variables used by the performance.

This ensures we don't run into discontinuities when reversing an
animation halfway through. I refactored AnimationValue to have knowledge
of the reverse curves and intervals.

We ended up with a bunch of different scrollable widgets and it was confusing.
This CL moves them all into one library to make them easier to find.

SnackBar, and PopupMenu instead of custom statuses.

This fixes some theoretical bugs whereby we were using hashCode to try
to get unique keys for objects, but really we wanted object identity.
It also lays the groundwork for a new GlobalKey concept.

I tried to keep the impact on the code minimal, which is why the "Key"
constructor is actually a factory that returns a StringKey. The code
has this class hierarchy:

```
   KeyBase
    |
   Key--------------+---------------+
    |               |               |
   StringKey    ObjectKey       UniqueKey
```

...where the constructors are Key and Key.stringify (StringKey),
Key.fromObjectIdentity (ObjectKey), and Key.unique (UniqueKey).

We could instead of factory methods use regular constructors with the
following hierarchy:

```
   KeyBase
    |
   LocalKey---------+---------------+
    |               |               |
   Key      ObjectIdentityKey   UniqueKey
```

...with constructors Key, Key.stringify, ObjectIdentityKey, and
UniqueKey, but I felt that that was maybe a more confusing hierarchy.
I don't have a strong opinion on this.

Route (named routes) no longer have a key, and have their own storage for their names.
RouseState no longer has a key, and uses an owner field pointing to a StatefulComponent instead.
As such, RouteBase no longer has a key.

HistoryEntry no longer uses a global int to ensure uniqueness.

Propagated this to stocks app.