The goal is to follow the guidelines in
https://github.com/flutter/engine/blob/master/sky/specs/style-guide.md#packages

Fixes #1638

Canvas is by far the most popular type of material by call site (and makes for
a faster default because it doesn't have a RRect clip).

These are now part of material.dart.

Also, have dialog buttons use the accent colour.
Also, generally rationalise some of this stuff to match Material better.

This patch makes the level of the ToolBar configurable. I've also cleaned up
the Tab code slightly.

For some reason, there's still a hairline between the ToolBar and the TabBar.
We might need to rethink how we draw the background a bit here.

Fixes #1454

In the vast majority of cases, folks should be interacting with the Widget
rather than its State.

Fixes #267

This widget is used in Material and Drawer. We don't currently support
animating towards null, but we can add that in a future patch.

Also, introduce Colors and Typography to hold the material colors and the
typography declarations. Previously we expected clients of these libraries to
import them into a namespace, but that doesn't play nice with re-exporting them
from material.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.

* 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