npm install gives this warning:
npm WARN deprecated object-keys@0.2.0: Please update to the latest object-keys
npm ls object-keys shows it as an indirect dependency of level-filesystem:
│ ├─┬ level-filesystem@1.2.0
│ │ └─┬ level-sublevel@5.2.3
│ │ └─┬ xtend@2.0.6
│ │ └── object-keys@0.2.0
Reading npm info shows that level-filesystem@1.2.0 is the current version, and depends on an outdated version of level-sublevel.