Turns out it gets nothing out of inheriting from AnimatedComponent.

Move the animation libraries into src/animation and change importers to use
package:sky/animation.dart. Also, move scheduler.dart into the animation
library so that the animation library can be self-contained.

These functions are now in sky:dart.

Code outside of package:sky should import this code using

package:sky/rendering.dart
package:sky/widgets.dart

Moving this code into the "src" directory is a convention that signifies that
and it cleans up the generated dartdoc because the libraries in the src
directory aren't included in the generated documentation. Instead, the classes
are documented in the widgets.dart and rendering.dart libraries.

Stop exporting framework.dart from basic.dart, since we now have widgets.dart exporting all of framework.dart.

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.

* Intentions => Behaviors
* SlideIn => SlideTransition
* FadeIn => FadeTransition
* removed unused AnimatedContainer intentions

This lets us be flexible in how to animate the properties of the container.
Currently used by SnackBar with a SlideIn intention.

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.

The idea is that AnimatedContainer is a drop-in replacement for Container that
provides implicit animations when its properties change.

R=mpcomplete@google.com