title: Stop Heroku from Pruning Dev Dependencies
body: <p>I have been trying to stop heroku from pruning my dev dependencies. Here is my config:
  My build script is: <code>"build": "NPM_CONFIG_PRODUCTION=false webpack -mode production"</code>
  And I have added <code>"heroku-postbuild": "npm run build"</code>
  My Dependencies:</p>
  
  <pre><code>"dependencies": {
      "express": "^4.16.3",
      "react": "^16.5.2",
      "react-dom": "^16.5.2"
    },
    "devDependencies": {
      "@babel/core": "^7.0.0",
      "@babel/plugin-proposal-class-properties": "^7.0.0",
      "@babel/preset-env": "^7.0.0",
      "@babel/preset-react": "^7.0.0",
      "babel-eslint": "^10.0.0",
      "babel-loader": "^8.0.0",
      "clean-webpack-plugin": "^0.1.19",
      "concurrently": "^4.0.0",
      "css-loader": "^1.0.0",
      "eslint": "^5.0.0",
      "eslint-config-airbnb": "^17.0.0",
      "eslint-plugin-import": "^2.11.0",
      "eslint-plugin-jsx-a11y": "^6.0.3",
      "eslint-plugin-react": "^7.7.0",
      "file-loader": "^2.0.0",
      "html-webpack-plugin": "^3.2.0",
      "nodemon": "^1.17.3",
      "style-loader": "^0.23.0",
      "url-loader": "^1.0.1",
      "webpack": "^4.5.0",
      "webpack-cli": "^3.0.8",
      "webpack-dev-server": "^3.1.3"
    }
  </code></pre>
  
  <p>I would appreciate any help with the config as I don't want to add my dev dependencies to my dependencies.</p>