test: async is still used in tests (#723)

Author: bcoe

package.json

@@ -53,6 +53,7 @@
     "@google-cloud/common": "^1.0.0",
     "@google-cloud/paginator": "^1.0.0",
     "@google-cloud/promisify": "^1.0.0",
+    "@types/protobufjs": "^6.0.0",
     "arrify": "^2.0.0",
     "compressible": "^2.0.12",
     "concat-stream": "^2.0.0",
@@ -91,6 +92,7 @@
     "@types/tmp": "0.1.0",
     "@types/uuid": "^3.4.4",
     "@types/xdg-basedir": "^2.0.0",
+    "async": "^3.0.0",
     "codecov": "^3.0.0",
     "eslint": "^5.0.0",
     "eslint-config-prettier": "^4.0.0",