Install fails when using local module and shrinkwrap. Because of missing file: in from key

When saving a local package to package.json it will add 'file:' as a prefix to the dependency.
"local-dependency": "file:plugin/local-dependency"

However, the installed dependency will only get "from" : "plugin/local-dependency".
Then when running npm shrinkwrap it only adds: "from": "plugin/local-dependency" to the dependency tree.

This will make the npm install from a npm-shrinkwrap.json fail with the error:
1388 error 404 'local-dependency' is not in the npm registry.

To reproduce:
npm install --save <some local dependency>
npm shrinkwrap
rm -rf node_modules
npm install

I have added some code that does make the npm install from npm-shrinkwrap.json work for me. Im not 100% sure about the implications of adding a "file:" as a prefix to the cache. However, all test should go run green.