Remove dart:utf in third_party.

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

 library inputstream;
 
 import 'dart:collection';
-import 'dart:utf';
+import 'package:utf/utf.dart';
 import 'package:source_maps/span.dart' show SourceFile;
 import 'char_encodings.dart';
 import 'constants.dart';
 import 'utils.dart';
 import 'encoding_parser.dart';
 
 /** Hooks to call into dart:io without directly referencing it. */
 class ConsoleSupport {
   List<int> bytesFromFile(source) => null;
 }
@@ skipping 286 matching lines @@
  * string doesn't correspond to a valid encoding.
  */
 String codecName(String encoding) {
   final asciiPunctuation = new RegExp(
       "[\u0009-\u000D\u0020-\u002F\u003A-\u0040\u005B-\u0060\u007B-\u007E]");
 
   if (encoding == null) return null;
   var canonicalName = encoding.replaceAll(asciiPunctuation, '').toLowerCase();
   return encodings[canonicalName];
 }