Refactor image handling in Sky

Refactor image handling in Sky to expose the loader and image as separate classes to Dart code. This makes it possible to avoid unnecessary paints, by only painting once when the image has loaded. Now that we've separated the loader and image classes, we can implement an image cache in Dart.

R=abarth@chromium.org, abarth

Committed: https://chromium.googlesource.com/external/mojo/+/ece46ba7a4b3de176e8a57d6082664b4dc9053ed

[abarth-chromium, 2015-06-01 22:57:10]

lgtm

https://codereview.chromium.org/1156003007/diff/20001/sky/engine/core/loader/...
File sky/engine/core/loader/ImageLoader.idl (right):

https://codereview.chromium.org/1156003007/diff/20001/sky/engine/core/loader/...
sky/engine/core/loader/ImageLoader.idl:9: void load(DOMString src,
ImageLoaderCallback callback);
Should ImageLoader support multiple in-flight image loads?  The interface looks
like it does but the implementation does not...

Maybe pass the |src| and the |callback| as constructor parameters?  That way it
is clear that each instance can handle only one load.

[jackson, 2015-06-01 23:10:26]

https://codereview.chromium.org/1156003007/diff/20001/sky/engine/core/loader/...
File sky/engine/core/loader/ImageLoader.idl (right):

https://codereview.chromium.org/1156003007/diff/20001/sky/engine/core/loader/...
sky/engine/core/loader/ImageLoader.idl:9: void load(DOMString src,
ImageLoaderCallback callback);
On 2015/06/01 22:57:09, abarth wrote:
> Should ImageLoader support multiple in-flight image loads?  The interface
looks
> like it does but the implementation does not...
> 
> Maybe pass the |src| and the |callback| as constructor parameters?  That way
it
> is clear that each instance can handle only one load.

Acknowledged.

[jackson, 2015-06-01 23:21:26]

Committed patchset #3 (id:40001) manually as
ece46ba7a4b3de176e8a57d6082664b4dc9053ed (presubmit successful).

[abarth-chromium, 2015-06-01 23:24:49]

https://codereview.chromium.org/1156003007/diff/40001/sky/engine/core/loader/...
File sky/engine/core/loader/ImageLoader.idl (right):

https://codereview.chromium.org/1156003007/diff/40001/sky/engine/core/loader/...
sky/engine/core/loader/ImageLoader.idl:9: void load();
What's the point of having a |load| function anymore?  It seems like we could
kick off the load during the constructor...  Maybe having a |cancel| method
would make more sense?

[jackson, 2015-06-02 02:11:19]

On 2015/06/01 at 23:24:49, abarth wrote:
>
https://codereview.chromium.org/1156003007/diff/40001/sky/engine/core/loader/...
> File sky/engine/core/loader/ImageLoader.idl (right):
> 
>
https://codereview.chromium.org/1156003007/diff/40001/sky/engine/core/loader/...
> sky/engine/core/loader/ImageLoader.idl:9: void load();
> What's the point of having a |load| function anymore?  It seems like we could
kick off the load during the constructor...  Maybe having a |cancel| method
would make more sense?

I'll kill |load| for now, we can add cancel some day