added Issue2857 for unit tests with basic tests for HTMLElement

tests/unit/issues/Issue2857.hx

@@ -0,0 +1,14 @@
+package unit.issues;
+
+class Issue2857 extends unit.Test {
+#if js
+	function testElement() {
+		var vid = js.Browser.document.createVideoElement();
+		t(Std.is(vid, js.html.VideoElement));
+		t(Std.is(vid, js.html.Element));
+		f(Std.is(vid, haxe.Http));
+		f(Std.is(vid, js.html.ArrayBuffer));
+		eq(Type.getClass(vid), js.html.VideoElement);
+	}
+#end
+}