Only load URL polyfill if needed

The URL polyfill is only required if window.URL lacks
a constructor.

'js-polyfills/url' does have its own feature detection but
relies on 'URL' being available at 'this.URL' which works
when loaded independently but not when included as a CommonJS
dependency.

Adding our own feature test here should make it easier to
avoid loading the polyfill entirely in future on browsers
that support the URL constructor.

h/static/scripts/polyfills.js

@@ -2,9 +2,13 @@
 require('core-js/es6/promise');
 require('core-js/fn/object/assign');
 
-// URL constructor, required by IE 10+,
-// Microsoft Edge.
-window.URL = require('js-polyfills/url').URL;
+// URL constructor, required by IE 10/11,
+// early versions of Microsoft Edge.
+try {
+  new window.URL('https://hypothes.is');
+} catch (err) {
+  window.URL = require('js-polyfills/url').URL;
+}
 
 // document.evaluate() implementation,
 // required by IE 10, 11