Implement ParentNode#ReplaceChildren.

Differential Revision: https://phabricator.services.mozilla.com/D75891

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1626015
gecko-commit: a3fcf899da1a3f92ee5ecd65ef5c038b8eb0729f
gecko-integration-branch: autoland
gecko-reviewers: emilio
diff --git a/dom/nodes/ParentNode-replaceChildren.html b/dom/nodes/ParentNode-replaceChildren.html
index 6557db4..b1c1008 100644
--- a/dom/nodes/ParentNode-replaceChildren.html
+++ b/dom/nodes/ParentNode-replaceChildren.html
@@ -94,14 +94,14 @@
 
       const observer = new MutationObserver(mutations => {
         t.step(() => {
-          assert_equals(phase, 1);
-          assert_equals(mutations.length, 1);
+          assert_equals(phase, 1, "phase");
+          assert_equals(mutations.length, 1, "mutations.length");
           const mutation = mutations[0];
-          assert_equals(mutation.type, "childList");
-          assert_equals(mutation.addedNodes.length, 2);
-          assert_array_equals([...mutation.addedNodes], insertions);
-          assert_equals(mutation.removedNodes.length, 2);
-          assert_array_equals([...mutation.removedNodes], children);
+          assert_equals(mutation.type, "childList", "mutation.type");
+          assert_equals(mutation.addedNodes.length, 2, "added nodes length");
+          assert_array_equals([...mutation.addedNodes], insertions, "added nodes");
+          assert_equals(mutation.removedNodes.length, 2, "removed nodes length");
+          assert_array_equals([...mutation.removedNodes], children, "removed nodes");
         });
         t.done();
       });