Make EfficientLength public, as EfficientLengthIterable.

runtime/lib/array_patch.dart

Index: runtime/lib/array_patch.dart
diff --git a/runtime/lib/array_patch.dart b/runtime/lib/array_patch.dart
index 109938bf9d2070ce26c6688a610ce310c8c88365..e422e306977d4a8859ed16ad2d110b75fb5eadc7 100644
--- a/runtime/lib/array_patch.dart
+++ b/runtime/lib/array_patch.dart
@@ -33,7 +33,7 @@ patch class List<E> {
   }
 
   /* patch */ factory List.from(Iterable elements, { bool growable: true }) {
-    if (elements is EfficientLength) {
+    if (elements is EfficientLengthIterable) {
       int length = elements.length;
       var list = growable ? new _GrowableList<E>(length) : new _List<E>(length);
       int i = 0;