Fixes a change in our RWC suite.

When comment emit got greatly improved recently, it shifted the responsibility of what node needed to have correct positions to actually retain comments in the tested scenario. With this change, the es2015 transform will now actually set the original node and positions on the block of a for of loop, if present, when it is transformed, so it can retain comments attached to its opening and closing tokens. Incidentally, the comment:

// The old emitter does not emit source maps for the block.
// We add the location to preserve comments.

seemed to indicate a TODO with respect to the sourcemaps, which I gather I have just done, by using updateBlock instead of always making a synthetic one.