Make sure to actually use yarn to install the main-packages, otherwise the packages.json#resolutions property will not be used and @babel/helpers would not get overruled

Overruled @babel/helpers to fix how initializers play with decorated properties. Thus circumventing the imperformant and lengthy code being generated by babel in the non-legacy option.

packages/babel-helpers/package.json

@@ -0,0 +1,21 @@
+{
+  "name": "@babel/helpers",
+  "version": "7.7.0",
+  "description": "Collection of helper functions used by Babel transforms.",
+  "author": "Sebastian McKenzie <sebmck@gmail.com>",
+  "homepage": "https://babeljs.io/",
+  "license": "MIT",
+  "publishConfig": {
+    "access": "public"
+  },
+  "repository": "https://github.com/babel/babel/tree/master/packages/babel-helpers",
+  "main": "lib/index.js",
+  "dependencies": {
+    "@babel/template": "^7.7.0",
+    "@babel/traverse": "^7.7.0",
+    "@babel/types": "^7.7.0"
+  },
+  "devDependencies": {
+    "@babel/helper-plugin-test-runner": "^7.0.0"
+  }
+}