Its actually a new feature that appears in 4.6.
its still sort of a work in progress, but its getting there.
“Turn this on using the new “Use Emissive For Static Lighting” option on shadow-casting components with Mobility set to “Static”. When enabled, Lightmass generates baked direct lighting from emissive texture sources.
Note: this is a fast but limited version of emissive lighting. Only direct lighting is computed but build time is not affected. It is best used in scenes that have other light sources.”