Switch transformers over to source_span

pkg/third_party/html5lib/lib/src/treebuilder.dart

 /// Internals to the tree builders.
 library treebuilder;
 
 import 'dart:collection';
 import 'package:html5lib/dom.dart';
 import 'package:html5lib/parser.dart' show getElementNameTuple;
-import 'package:source_maps/span.dart' show FileSpan;
+import 'package:source_span/source_span.dart';
 import 'constants.dart';
 import 'list_proxy.dart';
 import 'token.dart';
 import 'utils.dart';
 
 // The scope markers are inserted when entering object elements,
 // marquees, table cells, and table captions, and are used to prevent formatting
 // from "leaking" into tables, object elements, and marquees.
 const Node Marker = null;
 
@@ skipping 367 matching lines @@
   Document getDocument() => document;
 
   /// Return the final fragment.
   DocumentFragment getFragment() {
     //XXX assert innerHTML
     var fragment = new DocumentFragment();
     openElements[0].reparentChildren(fragment);
     return fragment;
   }
 }