Extend Iterable instead of implementing it.

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.
 
 // 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.
@@ skipping 186 matching lines @@
           "Invalid UTF16 at ${utf16CodeUnitIterator.position}");
     }
   }
 }
 
 /**
  * _ListRange in an internal type used to create a lightweight Interable on a
  * range within a source list. DO NOT MODIFY the underlying list while
  * iterating over it. The results of doing so are undefined.
  */
-class _ListRange implements Iterable {
+class _ListRange extends Iterable {
   final List _source;
   final int _offset;
   final int _length;
 
   _ListRange(source, [offset = 0, length]) :
       this._source = source,
       this._offset = offset,
       this._length = (length == null ? source.length - offset : length) {
     if (_offset < 0 || _offset > _source.length) {
       throw new RangeError.value(_offset);
@@ skipping 43 matching lines @@
     _offset -= by;
   }
 
   int get remaining => _end - _offset;
 
   void skip([int count = 1]) {
     _offset += count;
   }
 }