Extend Iterable instead of implementing it.

sdk/lib/utf/utf16.dart

Index: sdk/lib/utf/utf16.dart
diff --git a/sdk/lib/utf/utf16.dart b/sdk/lib/utf/utf16.dart
index d1bd012fc5992a3d4e33d684ad9515dd897d625f..431d695ad83a233f5e8130f82264945a07788d5e 100644
--- a/sdk/lib/utf/utf16.dart
+++ b/sdk/lib/utf/utf16.dart
@@ -215,7 +215,7 @@ typedef _ListRangeIterator _CodeUnitsProvider();
  * 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 IterableUtf16Decoder implements Iterable<int> {
+class IterableUtf16Decoder extends Iterable<int> {

[Ivan Posva, 2012/11/26 18:13:01]

Please remember by extending rather than implementing you add unnecessary type
arguments on the class extending the parameterized interface.

This ripples all the way down into the implementation causing you to allocate
two objects (object and its type arguments) for each new IterableUtf16Decoder.

[floitsch, 2012/11/28 14:16:01]

Added TODO.

[Lasse Reichstein Nielsen, 2012/11/28 21:29:35]

Is there any way that this could be made cheaper?
E.g., when the class and its superclass always have the same type
parameterization, and since type parameters are final on the object, could it be
stored only once?

   final _CodeUnitsProvider codeunitsProvider;
   final int replacementCodepoint;