title: Empty istanbul (nyc) report while coverage files are filled properly with Selenium
body: <p>The web app is written in python + typescript and uses Selenium for the font-end tests.
  I've tried to follow <a href="https://stackoverflow.com/questions/52747010/javascript-code-coverage-using-selenium-istanbul#">this answer</a> to setup the test coverage analysis. The simplified pipeline is:</p>
  <ol>
  <li>After TS --&gt; JS conversion the code is instrumented with nyc <code>nyc instrument ./dist/js_cov/ ./dist/js/ --embed-source true</code></li>
  <li>Selenium runs some tests and stores the output in the .nyc_output folder. The files are generated correctly with coverage info inside e.g.</li>
  </ol>
  <pre><code>{&quot;path/vars.bundle.js&quot;:{&quot;path&quot;:&quot;path/vars.bundle.js&quot;&quot;statementMap&quot;:{&quot;0&quot;:{&quot;start&quot;:{&quot;line&quot;:1,&quot;column&quot;:9},&quot;end&quot;:{&quot;line&quot;:550,&quot;column&quot;:1}},...
  </code></pre>
  <p>I've also checked that in the browser console <code>window.__coverage__</code> looks well with the list of files and coverage results.</p>
  <ol start="3">
  <li>I expect <code>nyc report --cwd .</code> to show the coverage result, but the report is always empty. Neither files nor coverage results are shown.</li>
  </ol>
  <pre><code>----------|---------|----------|---------|---------|-------------------
  File      | % Stmts | % Branch | % Funcs | % Lines | Uncovered Line #s
  ----------|---------|----------|---------|---------|-------------------
  All files |       0 |        0 |       0 |       0 |
  ----------|---------|----------|---------|---------|-------------------
  </code></pre>
  <p><code>nyc merge</code> works fine and create a general coverage file for all the tests, bu the report is still empty</p>
  <p>What could be the problem that causes an empty report?</p>