Remove dart:utf in third_party.

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

 /** Decodes bytes using the correct name. See [decodeBytes]. */
 library char_encodings;
 
 import 'dart:collection';
-import 'dart:utf';
+import 'package:utf/utf.dart';
 
 // TODO(jmesserly): this function is conspicuously absent from dart:utf.
 /**
  * Returns true if the [bytes] starts with a UTF-8 byte order mark.
  * Since UTF-8 doesn't have byte order, it's somewhat of a misnomer, but it is
  * used in HTML to detect the UTF-
  */
 bool hasUtf8Bom(List<int> bytes, [int offset = 0, int length]) {
   int end = length != null ? offset + length : bytes.length;
   return (offset + 3) <= end &&
@@ skipping 187 matching lines @@
       case 0x9D:
         if (replacementCodepoint == null) {
           throw new ArgumentError(
               "Invalid windows-1252 code point $char at $_offset");
         }
         return replacementCodepoint;
     }
     return char;
   }
 }