Extend Iterable instead of implementing it.

sdk/lib/utf/utf8.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.
 
 const int _UTF8_ONE_BYTE_MAX = 0x7f;
 const int _UTF8_TWO_BYTE_MAX = 0x7ff;
 const int _UTF8_THREE_BYTE_MAX = 0xffff;
 
 const int _UTF8_LO_SIX_BIT_MASK = 0x3f;
 
@@ skipping 109 matching lines @@
     int replacementCodepoint = UNICODE_REPLACEMENT_CHARACTER_CODEPOINT]) {
   return new Utf8Decoder(utf8EncodedBytes, offset, length,
       replacementCodepoint).decodeRest();
 }
 
 /**
  * Return type of [decodeUtf8AsIterable] 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 IterableUtf8Decoder implements Iterable<int> {
+class IterableUtf8Decoder extends Iterable<int> {
   final List<int> bytes;
   final int offset;
   final int length;
   final int replacementCodepoint;
 
   IterableUtf8Decoder(this.bytes, [this.offset = 0, this.length = null,
       this.replacementCodepoint = UNICODE_REPLACEMENT_CHARACTER_CODEPOINT]);
 
   Utf8Decoder iterator() => new Utf8Decoder(bytes, offset, length,
           replacementCodepoint);
@@ skipping 110 matching lines @@
     if (validSequence && nonOverlong && inRange) {
       return value;
     } else if (replacementCodepoint != null) {
       return replacementCodepoint;
     } else {
       throw new ArgumentError(
           "Invalid UTF8 at ${utf8EncodedBytesIterator.position - j}");
     }
   }
 }