[BUG] SourceMaps are not including the original *.ts files

[MSNev]

As part of investigating #2424 it was found that the source map files are not including the original *.ts files, but rather the rewritten *.js files from the build which have at least (there may be another one) been rewritten by the updateDistEsm script to replace the tslib references with the shims (to support IE)