title: rollup.JS and "'this' keyword is equivalent to 'undefined'
body: <p>I'm trying to bundle Angular2 modules using Rollup.js.
  this is my rollup.config.vendor.js file:</p>
  
  <pre><code>import typescript from 'rollup-plugin-typescript2';
  import resolve from 'rollup-plugin-node-resolve';
  import commonjs from 'rollup-plugin-commonjs';
  
  export default {
      entry: 'vendor.ts',
      dest: './Bundle/vendor.js',
      format: 'iife',
      moduleName: 'vendor',
      plugins: [
          typescript(),
          resolve({
              jsnext: true,
              main: true,
              browser: true
          }),
          commonjs({
              include: 'node_modules/rxjs/**',
          }),
      ]
  }
  </code></pre>
  
  <p>It creates a bundled js, but in the process it keeps printing this kind of message:</p>
  
  <pre><code>The 'this' keyword is equivalent to 'undefined' at the top level of an ES module, and has been rewritten
  https://github.com/rollup/rollup/wiki/Troubleshooting#this-is-undefined
  node_modules\@angular\forms\@angular\forms.es5.js (1:25)
  1: var __extends = (this &amp;&amp; this.__extends) || function (d, b) {
                              ^
  2:     for (var p in b) if (b.hasOwnProperty(p)) d[p] = b[p];
  3:     function __() { this.constructor = d; }
  </code></pre>
  
  <p>What does it mean?<br>
  Am I doing something wrong or is it the way it's supposed to be?</p>