title: Istanbul gives me coverage but ends output with an error
body: <p>I'm testing out a simple application (from the Mocha tutorial code here <a href="https://marcofranssen.nl/using-mocha-chai-sinon-to-test-node-js/" rel="noreferrer">https://marcofranssen.nl/using-mocha-chai-sinon-to-test-node-js/</a>) to try to get Istanbul to work. My problem is that Istanbul works fine to give me a coverage summary, but then spits out an error for some reason and I'm not sure why. My tests all pass so they're hopefully not the problem. Here is how I run Istanbul:</p>
  
  <pre><code>$ istanbul cover test.js
  
  =============================================================================
  Writing coverage object [C:\Users\path\test\coverage\coverage.json]
  Writing coverage reports at [C:\Users\path\test\coverage]
  =============================================================================
  
  =============================== Coverage summary ===============================
  
  Statements   : 54.55% ( 6/11 )
  Branches     : 100% ( 0/0 )
  Functions    : 0% ( 0/2 )
  Lines        : 54.55% ( 6/11 )
  ================================================================================
  ReferenceError: describe is not defined
      at Object.&lt;anonymous&gt; (C:\Users\path\test.js:9:386)
      at Module._compile (module.js:435:26)
      at Object.Module._extensions.(anonymous function) [as .js] (C:\Users\path
  \AppData\Roaming\npm\node_modules\istanbul\lib\hook.js:107:24)
      at Module.load (module.js:356:32)
      at Function.Module._load (module.js:311:12)
      at Function.Module.runMain (module.js:467:10)
      at runFn (C:\Users\path\AppData\Roaming\npm\node_modules\istanbul\lib\com
  mand\common\run-with-cover.js:122:16)
      at C:\Users\path\AppData\Roaming\npm\node_modules\istanbul\lib\command\co
  mmon\run-with-cover.js:251:17
      at C:\Users\path\AppData\Roaming\npm\node_modules\istanbul\lib\util\file-
  matcher.js:68:16
      at C:\Users\path\AppData\Roaming\npm\node_modules\istanbul\node_modules\a
  sync\lib\async.js:52:16
  </code></pre>
  
  <p>I'm not too sure why <code>describe</code> is not recognized as all the tests run fine and pass. </p>