Revert "Make EfficientLength public, as EfficientLengthIterable."

sdk/lib/core/iterable.dart

Index: sdk/lib/core/iterable.dart
diff --git a/sdk/lib/core/iterable.dart b/sdk/lib/core/iterable.dart
index c22283ab4e16da45383202539aa9494da16857f2..3de39bcdc989ef7f2276634bb611b2d33c891c5a 100644
--- a/sdk/lib/core/iterable.dart
+++ b/sdk/lib/core/iterable.dart
@@ -358,7 +358,7 @@ abstract class Iterable<E> {
    * Some iterables have a more efficient way to find the number of elements.
    */
   int get length {
-    assert(this is! EfficientLengthIterable);
+    assert(this is! EfficientLength);
     int count = 0;
     Iterator it = iterator;
     while (it.moveNext()) {
@@ -605,7 +605,7 @@ abstract class Iterable<E> {
 typedef E _Generator<E>(int index);
 
 class _GeneratorIterable<E> extends Iterable<E>
-                            implements EfficientLengthIterable<E> {
+                            implements EfficientLength {
   final int _start;
   final int _end;
   final _Generator<E> _generator;