Remove unused BidirectionalIterator from dart:_internal.

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 9a5f29fbfb70b26eb6da914a1bf3848b705319a9..f3e316611ceea8b0d2912436114c534a7416cd11 100644
--- a/sdk/lib/internal/iterable.dart
+++ b/sdk/lib/internal/iterable.dart
@@ -749,12 +749,6 @@ class EmptyIterator<E> implements Iterator<E> {
   E get current => null;
 }
 
-/** An [Iterator] that can move in both directions. */
-abstract class BidirectionalIterator<T> implements Iterator<T> {
-  bool movePrevious();
-}
-
-
 /**
  * Creates errors throw by [Iterable] when the element count is wrong.
  */