Revert "Switch libraries to using new tags."

sdk/lib/utf/utf_core.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-part of dart.utf;
-
 // TODO(jmesserly): would be nice to have this on String (dartbug.com/6501).
 /**
  * Provide a list of Unicode codepoints for a given string.
  */
 List<int> stringToCodepoints(String str) {
   // Note: str.charCodes gives us 16-bit code units on all Dart implementations.
   // So we need to convert.
   return _utf16CodeUnitsToCodepoints(str.charCodes);
 }
 
@@ skipping 221 matching lines @@
     _offset -= by;
   }
 
   int get remaining => _end - _offset;
 
   void skip([int count = 1]) {
     _offset += count;
   }
 }