Small fix to MappedListIterable

sdk/lib/internal/iterable.dart

Index: sdk/lib/internal/iterable.dart
diff --git a/sdk/lib/internal/iterable.dart b/sdk/lib/internal/iterable.dart
index 637b0218a00c3ddcd2a8fedf642c00c6c8ede38e..5ab7d4c6b1a351c66a11e00872b4f9343abb64d9 100644
--- a/sdk/lib/internal/iterable.dart
+++ b/sdk/lib/internal/iterable.dart
@@ -402,8 +402,7 @@ class MappedIterator<S, T> extends Iterator<T> {
  *
  * Expects efficient `length` and `elementAt` on the source iterable.
  */
-class MappedListIterable<S, T> extends ListIterable<T>
-                               implements EfficientLengthIterable<T> {
+class MappedListIterable<S, T> extends ListIterable<T> {
   final Iterable<S> _source;
   final _Transformation<S, T> _f;