sass-loader (as added in #18638) has a peer dependency on webpack ^2.0.0, but webpack 1.14.x is the latest permitted in package.json, causing installation errors:

npm ERR! argv "/usr/bin/node" "/usr/bin/npm" "i" 
npm ERR! node v4.7.3
npm ERR! npm  v2.15.11
npm ERR! code EPEERINVALID

npm ERR! peerinvalid The package webpack@1.14.0 does not satisfy its siblings' peerDependencies requirements!
npm ERR! peerinvalid Peer extract-text-webpack-plugin@1.0.1 wants webpack@^1.9.11
npm ERR! peerinvalid Peer stats-webpack-plugin@0.2.2 wants webpack@^1.0
npm ERR! peerinvalid Peer webpack-dev-middleware@1.8.4 wants webpack@1 || ^2.1.0-beta
npm ERR! peerinvalid Peer webpack-dev-server@1.16.3 wants webpack@>=1.3.0 <3
npm ERR! peerinvalid Peer babel-loader@6.2.10 wants webpack@1 || 2 || ^2.1.0-beta || ^2.2.0-rc
npm ERR! peerinvalid Peer sass-loader@6.0.2 wants webpack@^2.0.0                                 # <-- here

Some newer npm versions appear to ignore the broken peer dep, but this is still likely to cause an issue.

npm WARN sass-loader@6.0.2 requires a peer of webpack@^2.0.0 but none was installed.