Add constructors for SVG elements and remove static factory providers for html

tests/html/svgelement_test.dart

Index: tests/html/svgelement_test.dart
diff --git a/tests/html/svgelement_test.dart b/tests/html/svgelement_test.dart
index 6787d3cac47d2623022149820ad9f97710ac1fec..8a1ef5006644f926632152becd9917a9627f35c3 100644
--- a/tests/html/svgelement_test.dart
+++ b/tests/html/svgelement_test.dart
@@ -152,8 +152,8 @@ main() {
   group('outerHTML', () {
     test('outerHTML', () {
       final el = new SVGSVGElement();
-      el.elements.add(new SVGElement.tag("circle"));
-      el.elements.add(new SVGElement.tag("path"));
+      el.elements.add(new SVGCircleElement());
+      el.elements.add(new SVGPathElement());
       expect(el.outerHTML,
           '<svg version="1.1"><circle></circle><path></path></svg>');
     });
@@ -162,15 +162,15 @@ main() {
   group('innerHTML', () {
     test('get', () {
       final el = new SVGSVGElement();
-      el.elements.add(new SVGElement.tag("circle"));
-      el.elements.add(new SVGElement.tag("path"));
+      el.elements.add(new SVGCircleElement());
+      el.elements.add(new SVGPathElement());
       expect(el.innerHTML, '<circle></circle><path></path>');
     });
 
     test('set', () {
       final el = new SVGSVGElement();
-      el.elements.add(new SVGElement.tag("circle"));
-      el.elements.add(new SVGElement.tag("path"));
+      el.elements.add(new SVGCircleElement());
+      el.elements.add(new SVGPathElement());
       el.innerHTML = '<rect></rect><a></a>';
       expect(_nodeStrings(el.elements), ["rect", "a"]);
     });