[REVERT BEFORE MERGING] Extract the "decorate" helper to a new file

This has a few advantages:
1) GitHub highlights the helper code, making it easier to review
2) ESLint <3 (it caught an undeclared variable)
I also added flow types to help understanding what the functions do.

Thanks @Kovensky for the idea!

.eslintrc.json

@@ -28,6 +28,14 @@
       "env": {
         "jest": true
       }
+    },
+    {
+      "files": [
+        "packages/babel-helpers/src/helpers/*.js"
+      ],
+      "rules": {
+        "no-var": "off"
+      }
     }
   ]
 }

babel.config.js

@@ -50,6 +50,7 @@ module.exports = function(api) {
       ignoreLib ? "packages/*/lib" : null,
       "packages/babel-standalone/babel.js",
       "packages/babel-preset-env-standalone/babel-preset-env.js",
+      "packages/babel-helpers/src/helpers",
     ].filter(Boolean),
     presets: [["@babel/env", envOpts]],
     plugins: [