generated source maps are broken

Hi all,

I can't seem to generate valid sourcemaps when using gulp-babel, please let me know if I'm missing something or got something wrong, but I think this is a bug.

You can reproduce this behaviour here: https://github.com/Wonko7/gulp-babel-bug-report

Using gulp-babel 8.0.0-beta.2 to build a project produces incorrect sourcemaps, whereas calling babel manually on each js file works.

If you need anything else let me know.

Start with the linked reproduction repository and compare the source maps generated through gulp-babel 8.0.0-beta.2 with maps produced by calling Babel manually on each JavaScript file. Done means the gulp-babel output contains valid source maps matching the manual Babel behavior.

Written by the indexing model from the issue text.