fixed registration problem of service worker (#1807)

* fixed registration problem of service worker

* added a test for service worker

* reused some existing code to avoid code duplication

Co-authored-by: Jeffrey Warren <jeff@unterbahn.com>

test/core/sequencer/sw.js

@@ -0,0 +1,37 @@
+var setUpCache = new require('../../../examples/lib/cache')();
+var test = require('tape');
+
+function SWInstallation(){
+    return new Promise(() => {
+      return setupCache();
+    });
+}
+
+function UnRegisterSW(){
+
+    function unregister() {
+        return navigator.serviceWorker.getRegistrations()
+        .then(function(registrations) {
+            var unRegisteredWorker = registrations.map(function(registration) {
+            return registration.unregister();
+            });
+            return Promise.all(unRegisteredWorker);
+        });
+    }
+    
+    return Promise.all([
+        unregister(),
+        setUpCache.clearCache()
+    ]);
+}  
+
+test('Register service worker',function(t) {
+
+    t.test('unregister service worker',function(st) {
+      st.equal(UnRegisterSW(),true,'unregistered successfully and cleared the cache') 
+    })
+
+    t.test('install service worker',function(st) {
+      st.equal(SWInstallation(),true,'successfully installed new service worker')
+    });
+});