Switch transformers over to source_span

pkg/third_party/html5lib/test/parser_feature_test.dart

 /// Additional feature tests that aren't based on test data.
 library parser_feature_test;
 
 import 'package:unittest/unittest.dart';
 import 'package:html5lib/dom.dart';
 import 'package:html5lib/parser.dart';
 import 'package:html5lib/src/constants.dart';
 import 'package:html5lib/src/treebuilder.dart';
 
 main() {
@@ skipping 228 matching lines @@
   test('error printing without spans', () {
     var parser = new HtmlParser('foo');
     var doc = parser.parse();
     expect(doc.body.innerHtml, 'foo');
     expect(parser.errors.length, 1);
     expect(parser.errors[0].errorCode, 'expected-doctype-but-got-chars');
     expect(parser.errors[0].message,
         'Unexpected non-space characters. Expected DOCTYPE.');
     expect(parser.errors[0].toString(),
         'ParserError on line 1, column 4: Unexpected non-space characters. '
-        'Expected DOCTYPE.');
+          'Expected DOCTYPE.\n'
+        'foo\n'
+        '  ^');
   });
 
   test('Element.text', () {
     var doc = parseFragment('<div>foo<div>bar</div>baz</div>');
     var e = doc.firstChild;
     var text = e.firstChild;
     expect((text as Text).data, 'foo');
     expect(e.text, 'foobarbaz');
 
     e.text = 'FOO';
@@ skipping 23 matching lines @@
     expect((c as Comment).data, 'foo');
     expect(c.text, 'foo');
     expect(e.text, 'bar');
 
     c.text = 'qux';
     expect(c.data, 'qux');
     expect(c.text, 'qux');
     expect(e.text, 'bar');
   });
 }