Move dom-serializer.sky to dom_serializer.dart (and clean up some analyizer errors)

sky/tests/resources/dump-as-markup.sky

-<import src="dom-serializer.sky" as="DomSerializer"/>
 <script>
 import "dart:sky";
 import "dart:sky.internals" as internals;
+import "dom_serializer.dart" as DomSerializer;
 
 void _init(_) {
   window.addEventListener('load', (_) {
     internals.notifyTestComplete(DomSerializer.serializeNode(document));
   });
 }
 </script>