title: can't find source file when use istanbul to report TypeScript coverage with power-assert
body: <p>I want use Mocha, power-assert in my TypeScript project, when I use Chai, istanbul could generate Coverage HTML report, when I switch to power-assert, istanbul could only report coverage but cant generate HTML report with same config and exit with error. And if there is an error in test, the mocha will also broke.
  It sames a path issue in power-assert.
  I use nyc to report the coverage.</p>
  
  <p>My test command:</p>
  
  <pre><code>"test": "mocha --compilers ts:espower-typescript/guess test/*.ts",
  "test:coverage": "nyc npm test"
  </code></pre>
  
  <p>I'v set up a repo to reproduce this problem: <a href="https://github.com/LiJinyao/bug-power-assert" rel="nofollow noreferrer">https://github.com/LiJinyao/bug-power-assert</a></p>
  
  <p>Is there anyone have experience to set up a TypeScript test environment with code coverage using power-assert?</p>