Prevent another type of flash of unstyled content (#501)

Handles the case of a site is configured with a fully-qualified URL as a
static asset prefix.

packages/react-server/core/util/ClientCssHelper.js

@@ -12,23 +12,14 @@ module.exports = {
 		if (SERVER_SIDE) {
 			throw new Error("ClientCssHelper.registerPageLoad can't be called server-side");
 		}
-		const urlBase = location.protocol + "//" + location.host;
 
 		// for each css node in the head that the react-server server wrote to the response, note it down in the cache, so that
 		// we can remove it on a page to page transition.
 		var serverWrittenLinkNodes = document.head.querySelectorAll(`link[${PAGE_CSS_NODE_ID}],style[${PAGE_CSS_NODE_ID}]`);
 		for (var i = 0; i < serverWrittenLinkNodes.length; i++) {
 			var key, styleNode = serverWrittenLinkNodes[i];
 			if (styleNode.href) {
-				key = styleNode.href;
-
-				// The browser will give us a full URL even if we only put a
-				// path in on the server.  So, if we're comparing against just
-				// a path here we need to strip the base off to avoid a flash
-				// of unstyled content.
-				if (key.indexOf(urlBase) === 0) {
-					key = key.substr(urlBase.length);
-				}
+				key = normalizeLocalUrl(styleNode.href);
 			} else {
 				key = styleNode.innerHTML;
 			}
@@ -100,6 +91,20 @@ module.exports = {
 	},
 
 	_keyFromStyleSheet: function(style) {
-		return style.href || style.text;
+		return normalizeLocalUrl(style.href) || style.text;
 	},
 }
+
+function normalizeLocalUrl(url) {
+	const urlBase = location.protocol + "//" + location.host;
+
+	// The browser will give us a full URL even if we only put a
+	// path in on the server.  So, if we're comparing against just
+	// a path here we need to strip the base off to avoid a flash
+	// of unstyled content.
+	if (url && url.indexOf(urlBase) === 0) {
+		url = url.substr(urlBase.length);
+	}
+
+	return url;
+}