Switch transformers over to source_span

pkg/third_party/html5lib/lib/dom.dart

 /// A simple tree API that results from parsing html. Intended to be compatible
 /// with dart:html, but it is missing many types and APIs.
 library dom;
 
 // TODO(jmesserly): lots to do here. Originally I wanted to generate this using
 // our Blink IDL generator, but another idea is to directly use the excellent
 // http://dom.spec.whatwg.org/ and http://html.spec.whatwg.org/ and just
 // implement that.
 
 import 'dart:collection';
-import 'package:source_maps/span.dart' show FileSpan;
+import 'package:source_span/source_span.dart';
 
 import 'src/constants.dart';
 import 'src/css_class_set.dart';
 import 'src/list_proxy.dart';
 import 'src/query_selector.dart' as query;
 import 'src/token.dart';
 import 'src/tokenizer.dart';
 import 'dom_parsing.dart';
 import 'parser.dart';
 
@@ skipping 989 matching lines @@
 
 class _ConcatTextVisitor extends TreeVisitor {
   final _str = new StringBuffer();
 
   String toString() => _str.toString();
 
   visitText(Text node) {
     _str.write(node.data);
   }
 }