title: Webpack optimize with UglifyJS plugin causes runtime error
body: <p>I have an isomorphic React application that is bundled via webpack.</p>
  
  <p>I have 2 entry points corresponding to 2 bundled file outputs: <code>vendors.js</code> and <code>app.js</code>.</p>
  
  <p>When running the <em>webpack-dev-server</em> or when compiling without any optimization flags, everything works fine. However, as soon as I try to use the <em>Uglify</em> plugin, the compiled output contains errors.</p>
  
  <p>I have tried:</p>
  
  <pre><code>webpack -p
  webpack -optimize-minimize
  </code></pre>
  
  <p>or in the config:</p>
  
  <pre><code>new webpack.optimize.UglifyJsPlugin({sourceMap:false})
  </code></pre>
  
  <p>But all result in the same runtime error (undefined variables).</p>
  
  <p>Is there anything obvious that could be causing this? Is Uglify being over-zealous and removing something it shouldn't?</p>