title: Namespace gets overwritten using rollup generated files
body: <ul>
  <li>Rollup Version: 0.66.6</li>
  <li>Operating System (or Browser): Mac, Chrome</li>
  <li><p>Node Version: 10.13.0</p>
  
  <ol>
  <li>Generate script using rollup with this configuration</li>
  </ol></li>
  </ul>
  
  <pre><code>const outputFileNamespace = "Foo";
  
  const baseObject = {
      input: 'js/modules/',
      output: {
          name: outputFileNamespace,
          format: 'iife',
          sourcemap: true,
          banner: "/*Copyright text*/"
      },
      plugins: [(environment === PROD_ENV) &amp;&amp; uglify()]
  }; 
  
  </code></pre>
  2. Generate multiple files using this configuration : js1.js , js2.js
  3. Load these files in browser, namespace gets overriden
  
  <h3>Expected Behavior</h3>
  
  <p>If we need to generate multiple output files with the common global namespace as "Foo" here, there should be some check in the generated file which checks for the existence of a global namespace</p>
  
  <h3>Actual Behavior</h3>
  
  <p>Namespace gets overwritten and the latter one js functions are available </p>