Move image loading out of C++ into Dart

Move image loading out of C++ into Dart

We already know how to talk to the network_service from Dart
via fetch.dart.  Might as well use that for Image loading
as well insetad of having ImageLoader do it.

As part of this I've renamed *ImageLoader to *ImageDecoder
and moved all the image loading logic into Dart.  This required
me to teach the idl system about mojo handles so that I could
pass the resulting MojoHandle from fetch.dart up through to
ImageDecoder.

R=abarth@chromium.org, jackson@google.com, hansmuller@google.com

Committed: https://chromium.googlesource.com/external/mojo/+/9550a829ac7491844c78ea83987b670b9ea18ad5

[eseidel, 2015-06-09 18:26:32]

eseidel@chromium.org changed reviewers:
+ mpcomplete@chromium.org

[eseidel, 2015-06-09 18:26:33]

mpcomplete might have commentary on the idl changes.
jackson may have commentary on the image system.
hansmuller may care about my use of MojoHandle from dart.
And adam may just be opinionated. :p

[eseidel, 2015-06-09 18:55:48]

eseidel@chromium.org changed reviewers:
- hansmuller@google.com

[eseidel, 2015-06-09 18:55:49]

Bah, was confused.  Hans did the JS MojoHandle, etc, not the Dart ones, that was
zra.

[jackson, 2015-06-09 19:04:53]

lgtm

[abarth-chromium, 2015-06-09 19:06:55]

lgtm

https://codereview.chromium.org/1173703002/diff/1/sky/sdk/lib/framework/net/f...
File sky/sdk/lib/framework/net/fetch.dart (right):

https://codereview.chromium.org/1173703002/diff/1/sky/sdk/lib/framework/net/f...
sky/sdk/lib/framework/net/fetch.dart:24: Future<UrlResponse> fetchMojo(String
relativeUrl) async {
We need a better name for this function.

[eseidel, 2015-06-09 19:14:17]

Committed patchset #2 (id:20001) manually as
9550a829ac7491844c78ea83987b670b9ea18ad5 (presubmit successful).