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 dbc8c55d3ca7a0a9b39d41e8fd4bd02d7ab65be8..e435b1e7680786539216842297b416d9cc6541e1 100644
--- a/sdk/lib/core/iterable.dart
+++ b/sdk/lib/core/iterable.dart
@@ -347,7 +347,7 @@ abstract class Iterable<E> {
    * Some iterables have a more efficient way to find the number of elements.
    */
   int get length {
-    assert(this is! EfficientLength);
+    assert(this is! EfficientLengthIterable);
     int count = 0;
     Iterator it = iterator;
     while (it.moveNext()) {
@@ -594,7 +594,7 @@ abstract class Iterable<E> {
 typedef E _Generator<E>(int index);
 
 class _GeneratorIterable<E> extends Iterable<E>
-                            implements EfficientLength {
+                            implements EfficientLengthIterable<E> {
   final int _start;
   final int _end;
   final _Generator<E> _generator;