I think you'd be interested in Haxe. http://haxe.org/ Worth mentioning some popular Haxe libraries too... http://www.openfl.org/ http://www.haxeflixel.com/

Native level performance across desktop/server and mobile (and allows targetting the web, including WebGL and WIP support for Emscripten), open-source, reasonably easy to learn (pretty similar to ActionScript 3, so tutorials on Flash get you pretty close to understanding Haxe).

Worth a look IMO.