Fix source_map on windows.

The implementation of source map extraction did not work on Windows since it was introduced.

The issue was in the usual minor platform differences: in '\r\n' vs '\n' line endings, 'c:\' vs '/' absolute path detection, and '/' and '\' path separators. This fix normalized received data to sourcemapper on Windows to Unix form so that the mapper operates on Windows as well.

source_map tests checked to pass on Windows and Linux.