message: Override signature of document.implementation.hasFeature
diff: "kind": "indexer",
  "interface": "DOMStringMap",
  "signatures": ["[name: string]: string | undefined"]
  + },
  + {
  + "kind": "method",
  + "interface": "DOMImplementation",
  + "name": "hasFeature",
  + "signatures": ["hasFeature(feature: string | null, version: string | null): boolean"]
  }
  ]
  