Revert "Make EfficientLength extend Iterable."

sdk/lib/core/set.dart

Index: sdk/lib/core/set.dart
diff --git a/sdk/lib/core/set.dart b/sdk/lib/core/set.dart
index d2e9d8d14b03981e5ffb55b9b72d5a5e20f7316a..a4d7ed7ccb99f2f022b4c77a0c96e7ec11ae1fed 100644
--- a/sdk/lib/core/set.dart
+++ b/sdk/lib/core/set.dart
@@ -31,7 +31,7 @@ part of dart.core;
  * iterating either the set itself or any [Iterable] that is backed by the set,
  * such as the ones returned by methods like [where] and [map].
  */
-abstract class Set<E> extends EfficientLengthIterable<E> {
+abstract class Set<E> extends Iterable<E> implements EfficientLength {
   /**
    * Creates an empty [Set].
    *