Avoid testharness.js errors for assert_throws.

tests/wpt/mozilla/meta/MANIFEST.json

    "testharness"
   ],
   "mozilla/activation.html": [
-   "b4ebf61aea2934347bf2355375f6e2c53ef212bb",
+   "4e0514aa4dd8a59d35037c6966bb938ad5f49b6a",
    "testharness"
   ],
   "mozilla/adopt-panic.css": [
    "testharness"
   ],
   "mozilla/element_attribute.html": [
-   "9524405acb2345751b6dc1b160108ff665701d81",
+   "35625bedf3b323f8538174058c755cbccd25157c",
    "testharness"
   ],
   "mozilla/element_attributes.html": [
    "testharness"
   ],
   "mozilla/union.html": [
-   "47ee847e660eb907a7bd916cf37cf3ceba68ea7d",
+   "7a4ca34dd10a10ec6e013ac86038a070addf2889",
    "testharness"
   ],
   "mozilla/unitless-length.html": [

tests/wpt/mozilla/tests/mozilla/activation.html

@@ -42,7 +42,7 @@
       "Activate input");
 
     test(function () {
-      assert_throws(null, function () {
+      assert_throws("NotSupportedError", function () {
         var foo = document.getElementById("foo");
         foo.enterFormalActivationState();
       });

tests/wpt/mozilla/tests/mozilla/element_attribute.html

@@ -52,13 +52,13 @@
     });
 
     test(function() {
-      assert_throws(null, function () {
+      assert_throws("NamespaceError", function () {
         element.setAttributeNS("http://example.com", "xmlns", "foo");
       });
-      assert_throws(null, function () {
+      assert_throws("NamespaceError", function () {
         element.setAttributeNS("http://www.w3.org/2000/xmlns/", "attr", "value");
       });
-      assert_throws(null, function () {
+      assert_throws("NamespaceError", function () {
         element.setAttributeNS("http://www.w3.org/2000/xmlns/", "prefix:attr", "value");
       });
     });

tests/wpt/mozilla/tests/mozilla/union.html

@@ -16,7 +16,7 @@
   sel.add(optgroup, div);
   sel.add(opt, 5);
   sel.add(optgroup, 5);
-  assert_throws(null, function() { sel.add(div) });
+  assert_throws(new TypeError(), function() { sel.add(div) });
   sel.add(optgroup, function() {})
 });
 </script>