Extend Iterable instead of implementing it.

sdk/lib/utf/utf32.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.
 
 /**
  * Decodes the UTF-32 bytes as an iterable. Thus, the consumer can only convert
  * as much of the input as needed. Determines the byte order from the BOM,
  * or uses big-endian as a default. This method always strips a leading BOM.
  * Set the replacementCharacter to null to throw an ArgumentError
  * rather than replace the bad value.
@@ skipping 162 matching lines @@
       utf32EncodedBytes[offset + 2] == 0 && utf32EncodedBytes[offset + 3] == 0;
 }
 
 typedef Utf32BytesDecoder Utf32BytesDecoderProvider();
 
 /**
  * Return type of [decodeUtf32AsIterable] and variants. The Iterable type
  * provides an iterator on demand and the iterator will only translate bytes
  * as requested by the user of the iterator. (Note: results are not cached.)
  */
-class IterableUtf32Decoder implements Iterable<int> {
+class IterableUtf32Decoder extends Iterable<int> {
   final Utf32BytesDecoderProvider codeunitsProvider;
 
   IterableUtf32Decoder._(this.codeunitsProvider);
 
   Utf32BytesDecoder iterator() => codeunitsProvider();
 }
 
 /**
  * Abstrace parent class converts encoded bytes to codepoints.
  */
@@ skipping 116 matching lines @@
     value += (utf32EncodedBytesIterator.next() << 24);
     return value;
   }
 }
 
 bool _validCodepoint(int codepoint) {
   return (codepoint >= 0 && codepoint < UNICODE_UTF16_RESERVED_LO) ||
       (codepoint > UNICODE_UTF16_RESERVED_HI &&
       codepoint < UNICODE_VALID_RANGE_MAX);
 }