Revert "Bumps node to 8.9.4 (#16955)" (#16960)

This reverts commit 37429c0ab0.

.node-version

@@ -1 +1 @@
-8.9.4
+6.12.2

package.json

@@ -284,7 +284,7 @@
     "makelogs": "^4.0.3",
     "marked-text-renderer": "0.1.0",
     "mocha": "3.3.0",
-    "mock-fs": "^4.4.2",
+    "mock-fs": "4.2.0",
     "murmurhash3js": "3.0.1",
     "ncp": "2.0.0",
     "nock": "8.0.0",
@@ -310,7 +310,7 @@
     "yo": "2.0.0"
   },
   "engines": {
-    "node": "8.9.4",
+    "node": "6.12.2",
     "yarn": "^1.3.2"
   }
 }

yarn.lock

@@ -8411,9 +8411,9 @@ mocha@3.3.0:
     mkdirp "0.5.1"
     supports-color "3.1.2"
 
-mock-fs@^4.4.2:
-  version "4.4.2"
-  resolved "https://registry.yarnpkg.com/mock-fs/-/mock-fs-4.4.2.tgz#09dec5313f97095a450be6aa2ad8ab6738d63d6b"
+mock-fs@4.2.0:
+  version "4.2.0"
+  resolved "https://registry.yarnpkg.com/mock-fs/-/mock-fs-4.2.0.tgz#ef53ae17b77e64f67816dd0467f29208a3b26e19"
 
 module-not-found-error@^1.0.0:
   version "1.0.1"