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.

package.json

@@ -4,7 +4,7 @@
   "dependencies": {},
   "devDependencies": {
     "@babel/cli": "latest",
-    "@babel/core": "^7.6.2",
+    "@babel/core": "7.7.0",
     "@babel/plugin-proposal-class-properties": "latest",
     "@babel/plugin-proposal-decorators": "latest",
     "@babel/plugin-proposal-export-default-from": "latest",
@@ -37,5 +37,8 @@
     "build:csx-custom-elements": "cd ./packages/csx-custom-elements && npm run build",
     "watch:babel-transform-csx": "cd ./packages/babel-plugin-transform-csx-jsx && npm run watch",
     "watch:csx-custom-elements": "cd ./packages/csx-custom-elements && npm run watch"
+  },
+  "resolutions": {
+    "@babel/helpers": "file:./packages/babel-helpers"
   }
 }