Hoist knowledge of image_cache into widgets/basic.dart

Hoist knowledge of image_cache into widgets/basic.dart

Previously, RenderImage knew about image_cache and expected to work in terms of
URLs. Now RenderImage works directly with sky.Image and it's the job of the
widgets system to interact with the network cache.

At the widgets layer, I've factored this work into three parts:
1) A wrapper for RenderImage that works in terms of sky.Image.
2) A component that can deal with any sort of Future<sky.Image>.
3) A NetworkImage component that translates relative URLs into
   Future<sky.Image> using the image_cache.

A future CL will add a peer to NetworkImage that gets Future<sky.Image>s from
an asset bundle.

R=ianh@google.com, jackson@google.com

Committed: https://chromium.googlesource.com/external/mojo/+/f55c37fc136e652b84b7f3cbaed4170836d54b5c

[Hixie, 2015-07-01 20:53:34]

ianh@google.com changed reviewers:
+ ianh@google.com

[Hixie, 2015-07-01 20:53:34]

lgtm

https://codereview.chromium.org/1218023013/diff/1/sky/sdk/lib/widgets/basic.dart
File sky/sdk/lib/widgets/basic.dart (right):

https://codereview.chromium.org/1218023013/diff/1/sky/sdk/lib/widgets/basic.d...
sky/sdk/lib/widgets/basic.dart:495: void _fetchImage() {
fetch sounds like it hits the network

[jackson, 2015-07-01 21:17:07]

lgtm

[abarth-chromium, 2015-07-01 21:41:50]

Committed patchset #2 (id:20001) manually as
f55c37fc136e652b84b7f3cbaed4170836d54b5c (presubmit successful).