A preview of Adobe Extension Builder 3, a new developer tool that supports creating HTML5-based extensions, was launched in June at Adobe Labs. You can download it for free from here: http://labs.adobe.com/technologies/extensionbuilder3/
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.
Check out these videos to see how to build and debug HTML5 Extensions.