Fix error if URL is not available

Author: wooorm

lib/util/resolve.js

@@ -7,6 +7,7 @@ function resolve(h, url) {
     return ''
   }
 
+  /* istanbul ignore next - ignored for older Node */
   if (h.frozenBaseURL && typeof URL !== 'undefined') {
     return String(new URL(url, h.frozenBaseURL))
   }

test/index.js

@@ -90,6 +90,8 @@ test('fixtures', function(t) {
   t.end()
 
   function check(name) {
+    var ignore = /^base\b/.test(name) && typeof URL === 'undefined'
+
     t.test(name, function(st) {
       var input = String(
         fs.readFileSync(path.join(fixtures, name, 'index.html'))
@@ -128,6 +130,11 @@ test('fixtures', function(t) {
         assert(tree)
       }, 'should produce valid MDAST nodes')
 
+      if (ignore) {
+        st.end()
+        return
+      }
+
       if (!config || config.stringify !== false) {
         st.deepEqual(
           remark.stringify(tree),