Make it possible for sky-element to register any tag name

sky/framework/sky-element.sky

Index: sky/framework/sky-element.sky
diff --git a/sky/framework/sky-element.sky b/sky/framework/sky-element.sky
index 38d757a8f5a4fc45685ca4e1c9effd831b7675ca..8b04f79a5048e9db4bf0f0b5abfcd32df0616953 100644
--- a/sky/framework/sky-element.sky
+++ b/sky/framework/sky-element.sky
@@ -34,9 +34,7 @@ abstract class SkyElement extends Element {
 
   String get tagName => _getTagName(runtimeType);
 
-  // TODO(abarth): Rather than hard-coding "example" here, we should make the
-  // bindings read the |tagName| property of this object during construction.
-  SkyElement() : super("example") {
+  SkyElement() {
     created();
 
     // Invoke attributeChanged callback when element is first created too.