Implement DOM: prepend, append, before, after & replaceWith

LayoutTests/fast/dom/script-tests/childnode-replace-with.js

+function testReplaceWith(node, parent, type) {
+test(function() {
+assert_true("replaceWith" in node);
+assert_equals(typeof node.replaceWith, "function");
+assert_equals(node.replaceWith.length, 0);
+}, type + " should support replaceWith()");
+test(function() {
+var x;
+if (type == 'element' || type == 'document-fragment') {
+  x = document.createElement('x');
+}
+if (type == 'comment') {
+  x = document.createComment('x');
+}
+if (type == 'text') {
+  x = document.createTextNode('x');
+}
+parent.appendChild(node);
+assert_equals(node.parentNode, parent, "Appended node should have a parent");
+assert_array_equals(parentNode.childNodes, [node], "ParentNode should have 1 child node");
+node.replaceWith(x);
+assert_array_equals(parentNode.childNodes, [x], "ParentNode should have 1 child x");
+}, "replaceWith() should work for " + type);
+}