can't ignore optional deps

[yinghaochan]

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

What is the current behavior?
yarn add --ignore-optional installs optional deps

If the current behavior is a bug, please provide the steps to reproduce.

$ yarn add babel-cli --ignore-optional
yarn add v0.17.10
[1/4] Resolving packages...
[2/4] Fetching packages...
warning fsevents@1.0.15: The platform "linux" is incompatible with this module.
info "fsevents@1.0.15" is an optional dependency and failed compatibility check. Excluding it from installation.

from yarn.lock

babel-cli@^6.18.0:
  version "6.18.0"
  resolved "https://registry.yarnpkg.com/babel-cli/-/babel-cli-6.18.0.tgz#92117f341add9dead90f6fa7d0a97c0cc08ec186"
  dependencies:
    babel-core "^6.18.0"
    babel-polyfill "^6.16.0"
    babel-register "^6.18.0"
    babel-runtime "^6.9.0"
    commander "^2.8.1"
    convert-source-map "^1.1.0"
    fs-readdir-recursive "^1.0.0"
    glob "^5.0.5"
    lodash "^4.2.0"
    output-file-sync "^1.1.0"
    path-is-absolute "^1.0.0"
    slash "^1.0.0"
    source-map "^0.5.0"
    v8flags "^2.0.10"
  optionalDependencies:
    chokidar "^1.0.0"


chokidar@^1.0.0:
  version "1.6.1"
  resolved "https://registry.yarnpkg.com/chokidar/-/chokidar-1.6.1.tgz#2f4447ab5e96e50fb3d789fd90d4c72e0e4c70c2"
  dependencies:
    anymatch "^1.3.0"
    async-each "^1.0.0"
    glob-parent "^2.0.0"
    inherits "^2.0.1"
    is-binary-path "^1.0.0"
    is-glob "^2.0.0"
    path-is-absolute "^1.0.0"
    readdirp "^2.0.0"
  optionalDependencies:
    fsevents "^1.0.0"

What is the expected behavior?
optional deps and optional deps in children will not be isntalled

Please mention your node.js, yarn and operating system version.
LTS latest, Ubuntu 14.04 LTS (GNU/Linux 3.13.0-24-generic x86_64)

[sebmck]

#2116 should have fixed this.