Package-lock.json missing some deps compared to npm's version

#### Issue description:
After converting yarn.lock, generated package-lock.json is missing some dep declaration and some "dev" flags (using 1.9.4)

Here all json/lock files : https://gofile.io/uploadFiles
Or directly attached (beware of .txt extension)
[package-lock-npm.json.txt](https://github.com/imsnif/synp/files/5341805/package-lock-npm.json.txt)
[package-lock-synp.json.txt](https://github.com/imsnif/synp/files/5341806/package-lock-synp.json.txt)
[package.json.txt](https://github.com/imsnif/synp/files/5341807/package.json.txt)
[yarn.lock.txt](https://github.com/imsnif/synp/files/5341808/yarn.lock.txt)

I made a gitlab project for testing synp AND https://github.com/jeremylong/DependencyCheck.
https://gitlab.com/boloss/test-dependency-check
You can fork it if you want

Start by comparing the attached package-lock-npm.json.txt, package-lock-synp.json.txt, package.json.txt, and yarn.lock.txt, then reproduce the conversion with synp 1.9.4. Investigate why dependency declarations and dev flags differ, and consider the issue complete when the generated package-lock.json matches npm's version for the supplied project.

Written by the indexing model from the issue text.