Update Button to be made of Material

Update Button to be made of Material

To accomplish this, I made the following changes:

1) Material is now in charge of drawing the material shadows.
2) In order to mix in the style for the shadow, Element now takes a list of
   Styles instead of a single style.
3) Update all clients of Element#style to understand that we now have a list.
4) Update components that drawer shadows to have Material do that work instead.
   a) One exception: FloatingActionButton draws its own shadow because of its
      crazy clip requirements. We'll probably want to find a better way for
      FloatingActionButton to clip in the future.

I've also added a widgets-fn example to demo the fn material widgets.

This CL introduces a bug into Drawer whereby you can get ink splashes
everywhere in the drawer. In the future, we'll need to separate out the
different material aspects to get non-splashable materials.

R=rafaelw@chromium.org

Committed: https://chromium.googlesource.com/external/mojo/+/7d249a32158add826b8f9d275689925a9ef6f31b

[rafaelw, 2015-03-11 21:17:15]

lgtm. nice.

https://codereview.chromium.org/1003553002/diff/1/sky/framework/fn.dart
File sky/framework/fn.dart (right):

https://codereview.chromium.org/1003553002/diff/1/sky/framework/fn.dart#newco...
sky/framework/fn.dart:147: _class = '';
bit:

_class = styles == null ? '' : styles.map((s) => s._className).join(' ');

(or something like that).

[abarth-chromium, 2015-03-11 21:20:05]

https://codereview.chromium.org/1003553002/diff/1/sky/framework/fn.dart
File sky/framework/fn.dart (right):

https://codereview.chromium.org/1003553002/diff/1/sky/framework/fn.dart#newco...
sky/framework/fn.dart:147: _class = '';
On 2015/03/11 at 21:17:15, rafaelw wrote:
> bit:
> 
> _class = styles == null ? '' : styles.map((s) => s._className).join(' ');
> 
> (or something like that).

Done.

[abarth-chromium, 2015-03-11 21:20:16]

Committed patchset #1 (id:1) manually as
7d249a32158add826b8f9d275689925a9ef6f31b (presubmit successful).