Work around broken URL constructor in PhantomJS 2.x

Under PhantomJS 2.x `new URL(uri)` works, so the feature test in
`polyfills.js` passes, but the resulting URL object is an apparently
empty object.

Properties such as `hostname` and `href` are missing for example.

Load the URL polyfill explicitly in the Karma-specific polyfill module.

h/static/scripts/karma-phantomjs-polyfill.js

@@ -19,6 +19,10 @@ require('raf').polyfill();
 // app itself.
 require('./polyfills');
 
+// PhantomJS 2.x includes a `URL` constructor so `new URL` works
+// but it appears to be broken.
+require('js-polyfills/url');
+
 // disallow console output during tests
 ['debug', 'log', 'warn', 'error'].forEach(function (method) {
   var realFn = window.console[method];