I've been thinking. Do we ever want to set the whole source content while we are generating a source and map piece by piece? It seems like you would just add it to the map at the end, or you would want to add the snippet of original source that the generated source comes from and the source content could be built up the same way that the generated code is.

Any opinions on the latter? cc @michaelficarra 

I'm inclined to say if we don't do that, we should just remove the source content stuff from `SourceNode` and let people add it to the map directly after they call `toStringWithSourceMap`.