Make EfficientLength public, as EfficientLengthIterable.

runtime/lib/growable_array.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 class _GrowableList<T> extends ListBase<T> {
 
   void insert(int index, T element) {
     if ((index < 0) || (index > length)) {
       throw new RangeError.range(index, 0, length);
     }
@@ skipping 139 matching lines @@
     this[len] = value;
   }
 
   void addAll(Iterable<T> iterable) {
     var len = length;
     final cid = ClassID.getID(iterable);
     final isVMList =
         (cid == ClassID.cidArray) ||
         (cid == ClassID.cidGrowableObjectArray) ||
         (cid == ClassID.cidImmutableArray);
-    if (isVMList || (iterable is EfficientLength)) {
+    if (isVMList || (iterable is EfficientLengthIterable)) {
       var cap = _capacity;
       // Pregrow if we know iterable.length.
       var iterLen = iterable.length;
       var newLen = len + iterLen;
       if (newLen > cap) {
         do {
           cap *= 2;
         } while (newLen > cap);
         _grow(cap);
       }
@@ skipping 165 matching lines @@
       result._setLength(length);
       return result;
     }
     return growable ? <T>[] : new List<T>(0);
   }
 
   Set<T> toSet() {
     return new Set<T>.from(this);
   }
 }