WIP: make <a> tags SVG elements when they have xlink:href.

Needs a test case.

packages/ui/render.js

@@ -347,7 +347,21 @@ var materialize = function (node, parent, before, parentComponent) {
   } else if (node instanceof HTML.Tag) {
     var tagName = node.tagName;
     var elem;
-    if (HTML.isKnownSVGElement(tagName) && document.createElementNS) {
+    if ((HTML.isKnownSVGElement(tagName) ||
+         // We generally aren't able to detect SVG <a> elements because
+         // if "A" were in our list of known svg element names, then all
+         // <a> nodes would be created using
+         // `document.createElementNS`. But in the special case of <a
+         // xlink:href="...">, we can at least detect that attribute and
+         // create an SVG <a> tag in that case.
+         //
+         // However, we still have a general problem of knowing when to
+         // use document.createElementNS and when to use
+         // document.createElement; for example, font tags will always
+         // be created as SVG elements which can cause other
+         // problems. #1977
+         (tagName === "A" && node.attrs["xlink:href"]) !== undefined) &&
+        document.createElementNS) {
       elem = document.createElementNS('http://www.w3.org/2000/svg', tagName);
     } else {
       elem = document.createElement(node.tagName);