Do you want to request a feature or report a bug?
bug

What is the current behavior?

yarn --ignore-optional doesn't install all (none optional) dependencies

Related to #4448

Steps to reproduce:
package.json:
{ "dependencies": { "some-package": "^4.1.0", "yargs": "^9.0.1" } }

now some-package has this as optionalDependency:
"yargs": "^7.0.2"

the result of yarn --ignore-optional is missing package: is-fullwidth-code-point
which dependency path is: yargs -> string-width -> is-fullwidth-code-point

Tested on:
node 6.11.2
yarn 1.3.2