We tried to track the source text for TSR/DSR information in the Frame, but that conflated two different semantics for a frame -- on one hand it was a holder for the template/transclusion arguments, used to determine (eg) how {{{1}}} was evaluated, and on the other hand it was holding the source text for source range (TSR/DSR) information.

Split these two meanings by adding a Source reference directly to the SourceRange/DomSourceRange.

Initially this will be optional, since it is not serialized in data-parsoid and thus unavailable for html2wt. Eventually we may figure out an efficient encoding which lets us serialize/deserialize this efficiently, and then we'd expect to make this mandatory.