dom: implement NonDocumentTypeChildNode mixin for element

tests/wpt/dom/nodes/Element-previousElementSibling-svg.svg

@@ -0,0 +1,28 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<svg xmlns="http://www.w3.org/2000/svg"
+     xmlns:h="http://www.w3.org/1999/xhtml"
+     version="1.1"
+     width="100%" height="100%" viewBox="0 0 400 400">
+<title>previousElementSibling</title>
+<h:script src="/resources/testharness.js"/>
+<h:script src="/resources/testharnessreport.js"/>
+
+<text x="200" y="40" font-size="25" fill="black" text-anchor="middle">Test of previousElementSibling</text>
+<text id="parentEl" x="200" y="70" font-size="20" fill="black" text-anchor="middle">The result of <tspan id="first_element_child">this test</tspan> is
+<tspan id="middle_element_child" font-weight="bold">unknown.</tspan>
+
+
+
+<tspan id="last_element_child" display="none">fnord</tspan> </text>
+
+<h:script><![CDATA[
+test(function() {
+  var parentEl = document.getElementById("parentEl");
+  var lec = document.getElementById("last_element_child");
+  var pes = lec.previousElementSibling;
+  assert_true(!!pes)
+  assert_equals(pes.nodeType, 1)
+  assert_equals(pes.getAttribute("id"), "middle_element_child")
+})
+]]></h:script>
+</svg>