Remove dart:utf in third_party.

pkg/third_party/html5lib/test/tokenizer_test.dart

 library tokenizer_test;
 
 // Note: mirrors used to match the getattr usage in the original test
 import 'dart:async';
 import 'dart:io';
 import 'dart:json' as json;
 import 'dart:mirrors';
-import 'dart:utf';
 import 'package:path/path.dart' as pathos;
 import 'package:unittest/unittest.dart';
 import 'package:html5lib/src/char_encodings.dart';
 import 'package:html5lib/src/constants.dart' as constants;
 import 'package:html5lib/src/token.dart';
 import 'package:html5lib/src/tokenizer.dart';
 import 'package:html5lib/src/utils.dart';
+import 'package:utf/utf.dart';
 import 'support.dart';
 
 class TokenizerTestParser {
   String _state;
   var _lastStartTag;
   List outputTokens;
 
   TokenizerTestParser(String initialState, [lastStartTag])
       : _state = initialState,
         _lastStartTag = lastStartTag;
@@ skipping 235 matching lines @@
         for (var initialState in testInfo["initialStates"]) {
           test(testInfo["description"], () {
             testInfo["initialState"] = camelCase(initialState);
             runTokenizerTest(testInfo);
           });
         }
       }
     });
   }
 }