make our openfl compile/work headlessly on nodejs (for testing)

[nadako]

known places:

• Timer - depends on window.set/clearInterval
• Bitmap - depends on some browser API I can't remember now (probably canvas?)

[nadako]

To ensure that this works we should also add some tests to our openfl's test suite that's already running on nodejs.

[nadako]

Implementation-wise this is interesting. The easiest way is to sprinkle some #if guards, but in the long term, I think it's better to move the platform-specific implementation code into a separate class path, which is a good approach in general and will allow us to support different platforms easier.