title: webpack.optimize.UglifyJSPlugin does not work while uglifyjs-webpack-plugin works
body: <p>I have 2 files <code>app.js</code> &amp; <code>math.js</code></p>
  
  <p>It contains the following code -</p>
  
  <h1>app.js</h1>
  
  <pre><code>import { sum } from "./math";
  
  console.log("");
  
  console.log(sum(2, 3));
  </code></pre>
  
  <h1>math.js</h1>
  
  <pre><code>export const sum = (a, b) =&gt; a + b;
  export const multiply = (m, n) =&gt; m * n;
  </code></pre>
  
  <h1>webpack.config.js</h1>
  
  <pre><code>const path = require("path");
  const webpack = require("webpack");
  const UglifyJsPlugin = require("uglifyjs-webpack-plugin");
  const config = {
      entry: "./app.js",
      output: {
          path: path.resolve(__dirname, "dist"),
          filename: "bundle.js"
      },
      plugins: [
          new UglifyJsPlugin()
          // new webpack.optimize.UglifyJsPlugin() doesn't work
      ]
  };
  
  module.exports = config;
  </code></pre>
  
  <p>The complete code is available at <a href="https://github.com/deadcoder0904/webpack-treeshake" rel="nofollow noreferrer">https://github.com/deadcoder0904/webpack-treeshake</a></p>
  
  <p>The problem is <code>UglifyJsPlugin</code> works but <code>webpack.optimize.UglifyJsPlugin()</code> doesn't work</p>
  
  <p>My <code>webpack</code> version is v3.11.0</p>