I think the issue is that your manifest version is 2.0, the dark icons were not added to the manifest schema until 3.0. Extension Buidler is validating your manifest against the 2.0 schema, which is why it's complaining. If you change the extension manifest version to 3.0 (in the ExtensionManifest tag) then I would expect those warnings to go away. It looks like your extension targets CS5 - CS6, in theory the CSXS version in CS5 could be tripped up by using CSXS 3 (CS 6) manifest, however I believe that CSXS will just ignore any elements it doesn't understand (e.g. the dark icon tags).
Hope that helps!
Hi Louis, thanks for the info!
I just tested the 3.0 manifest in CS5/5.5, and it causes the extension to not appear. So it seems like I'll have to build separate versions for CS5 & CS6, and bundle the two builds together (bloating the final package in the process).
I haven't been able to test the icons in CS6 yet, as I am affected by this bug:
http://forums.adobe.com/message/4987214 (which hasn't been addressed)
Furthermore, from: http://forums.adobe.com/thread/1113624
Some extensions do not have a Retina ready icon and thus will be blank when you have the panel minimized. In most cases like these, we may have to wait until the manufacturer also releases a HiDPI version of the Extension.
Where is the documentation for this? Why are statements like this being made by staff (putting it on the extension developers), when Adobe has released NO implentation details—afaik, it's not even supported. With all due respect, the developer ecosystem for extensions is hideous.