I would like to make a 2D game with one codebase that runs on iPhone, Android, and web (any flavour of web -- HTML5, Flash, Silverlight, etc.)

What are my options? I should be able to write my code once, and run it anywhere. I also absolutely need the ability to write unit tests (or write a unit-testing framework) -- I cannot make sizable games without testing.

Unity is good, but unity is 3d; even with hacks, the graphical assets will still be 3d. I'm after 2d, not 3d.

(If you need a Mac or separate licensing for the Mac part, that's okay.)