Add xhr support for render file in browser.

src/util/url.js

@@ -0,0 +1,23 @@
+const resolve = require('resolve-url')
+const splitPathRe = /^(\/?|)([\s\S]*?)((?:\.{1,2}|[^/]+?|)(\.[^./]*|))(?:[/]*)$/
+const urlRe = /^(?:\w+:)?\/\/([^\s.]+\.\S{2}|localhost[:?\d]*)\S*$/
+
+// https://github.com/jinder/path/blob/master/path.js#L567
+exports.extname = function (path) {
+  return splitPathRe.exec(path).slice(1)[3]
+}
+
+// https://www.npmjs.com/package/is-url
+exports.valid = function (path) {
+  return urlRe.test(path)
+}
+
+exports.resolve = function (root, path) {
+  if (Object.prototype.toString.call(root) === '[object Array]') {
+    root = root[0]
+  }
+  if (root && root.charAt(root.length - 1) !== '/') {
+    root += '/'
+  }
+  return resolve(root, path)
+}