Pin dependencies (#780)

Author: renovate[bot]

src/React.Core/package-lock.json

@@ -6,8 +6,8 @@
     "build": "webpack"
   },
   "devDependencies": {
-    "@babel/core": "^7.4.3",
-    "@babel/standalone": "^7.4.3",
+    "@babel/core": "7.4.3",
+    "@babel/standalone": "7.4.3",
     "prop-types": "15.7.2",
     "react": "16.8.6",
     "react-dom": "16.8.6",