Flash will be out & HTML5 is in as the cross-app, cross-platform foundation for extending Creative Cloud apps:
When Creative Cloud was launched in June 2013, we added support for HTML5-based panels in several Adobe products… Since June, support for HTML5-based panels has been added to more Adobe products. As of now Photoshop CC, Illustrator CC, Premiere Pro CC and Prelude CC support HTML5-based panels.
A preview of Adobe Extension Builder 3, a new developer tool that supports creating HTML5-based extensions, was launched in June at Adobe Labs.
Starting in the middle of 2014 we will begin removing Flash-based extension support in Creative Cloud products, starting with Photoshop CC. We encourage all developers to start planning and migrating to the new extensibility framework based on HTML5.
Here the excellently named Hallgrimur Th. Bjornsson shows how to create a couple of panels. You can follow more from his team via their blog.