In the Multisample Coverage investigation (#267) there was an agreement to emulate the sampleMask behaviour using the shader-output mask in Metal.

There were some concerns that there would be a problem with using both the alphaToCoverage and sampleMask at the same time, since some APIs are not allowing usage of both the alphaToCoverage and shader-output mask at the same time. The Metal spec seemed to say that it can be done there, but after some testing in Dawn I think it's not the case.

A possible workaround that was mentioned earlier was to implement alpha-to-coverage inside shaders too.