Adds Node-compatible protocolForURL

The current implementation of the DOM helper’s `protocolForURL` relies
on the implicit parsing and normalization that the browser’s DOM does
when setting the A element’s `href` property.

However, the DOM helper is often used with subset DOM implementations
like simple-dom. In those cases, we cannot rely on this normalization.

This commit detects when URL parsing doesn’t happen when setting the
`href` property on an `A` element and falls back to using Node’s
`URL` package.

packages/dom-helper/lib/main.js

@@ -1,3 +1,5 @@
+/*globals module*/
+
 import Morph from "./htmlbars-runtime/morph";
 import AttrMorph from "./morph-attr";
 import {
@@ -565,15 +567,39 @@ prototype.parseHTML = function(html, contextualElement) {
 };
 
 var parsingNode;
+var URL;
 
-// Used to determine whether a URL needs to be sanitized.
 prototype.protocolForURL = function(url) {
+  // Test to see if our DOM implementation parses
+  // and normalizes URLs.
+  var protocol = browserProtocolForURL.call(this, 'foobar:baz');
+  if (protocol === 'foobar:') {
+    // Swap in the method that doesn't do this test now that
+    // we know it works.
+    prototype.protocolForURL = browserProtocolForURL;
+  } else {
+    // Otherwise, we need to fall back to our own URL parsing.
+    // Global `require` is shadowed by Ember's loader so we have to use the fully
+    // qualified `module.require`.
+    URL = module.require('url');
+    prototype.protocolForURL = nodeProtocolForURL;
+  }
+
+  return this.protocolForURL(url);
+};
+
+function browserProtocolForURL(url) {
   if (!parsingNode) {
     parsingNode = this.document.createElement('a');
   }
 
   parsingNode.href = url;
   return parsingNode.protocol;
-};
+}
+
+function nodeProtocolForURL(url) {
+  var protocol = URL.parse(url).protocol;
+  return (protocol === null) ? ':' : protocol;
+}
 
 export default DOMHelper;