title: Service Worker fetch event is not being triggered
body: <pre><code>      self.addEventListener('install', function (event) {
      console.log('SW Installed');
      event.waitUntil(
        caches.open('static')
          .then(function (cache) {
            cache.addAll([
              '/',
              '/static/js/renderjson.js',
              "/static/css/app.css",
              '/static/js/contribute.js',
              "http://pending.biothings.io/denovodb/metadata",
              "http://pending.biothings.io/ccle/metadata",
              "http://pending.biothings.io/kaviar/metadata",
              "http://pending.biothings.io/biomuta/metadata",
              "http://pending.biothings.io/fire/metadata",
            ]);
          })
      );
    });
  
    self.addEventListener('activate', function () {
      console.log('SW Activated');
    });
  
    self.addEventListener('fetch', function(event) {
      console.log('FETCH',event.request);
      event.respondWith(
        caches.match(event.request)
          .then(function(response) {
            if (response) {
                  // retrieve from cache
                  console.log('Found ', event.request.url, ' in cache');
                  return response;
              }
  
              // if not found in cache, return default offline content (only if this is a navigation request)
              if (event.request.mode === 'navigate') {
                  return caches.match('/');
              }
  
              // fetch as normal
              console.log('Network request for ', event.request.url);
              return fetch(event.request);
          })
      );
    });
  </code></pre>
  
  <p>I'm new to PWA's so bear with me, I'm running a tornado server here is the registration:</p>
  
  <pre><code>if ('serviceWorker' in navigator) {
    // Use the window load event to keep the page load performant
    window.addEventListener('load', () =&gt; {
      navigator.serviceWorker.register('/static/js/worker.js');
      console.log('Service Worker registered');
    });
  }
  </code></pre>
  
  <p>I put a few console logs to see if the fetch event was being triggered but nothing seems to be happening, when I check the offline box on the Application tab and refresh the app does not work offline. All the assets are cached correctly and verified they are there. Looks like the last bit is just making the Worker server the right cached files. </p>