Make EfficientLength public, as EfficientLengthIterable.

sdk/lib/collection/iterable.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.
 
 part of dart.collection;
 
 /**
  * This [Iterable] mixin implements all [Iterable] members except `iterator`.
  *
  * All other methods are implemented in terms of `iterator`.
@@ skipping 72 matching lines @@
     }
     return false;
   }
 
   List<E> toList({ bool growable: true }) =>
       new List<E>.from(this, growable: growable);
 
   Set<E> toSet() => new Set<E>.from(this);
 
   int get length {
-    assert(this is! EfficientLength);
+    assert(this is! EfficientLengthIterable);
     int count = 0;
     Iterator it = iterator;
     while (it.moveNext()) {
       count++;
     }
     return count;
   }
 
   bool get isEmpty => !iterator.moveNext();
 
@@ skipping 286 matching lines @@
       elision = "...";
       length += ELLIPSIS_SIZE + OVERHEAD;
     }
   }
   if (elision != null) {
     parts.add(elision);
   }
   parts.add(penultimateString);
   parts.add(ultimateString);
 }
+
+
+/**
+ * Marker interface for [Iterable] implementations that have an efficient
+ * [length] getter.
+ *
+ * An iterable implementing this interface should have an "efficient"
+ * implementation of `length` that doesn't trigger iteration of the
+ * iterable. Being efficient doesn't necessarily require being constant
+ * time, but should at least attempt to have have execution time that is
+ * better than linear in the elements of the iterable.
+ *
+ * Methods that worry about reading the length of an `Iterable` because it
+ * may be inefficient or may trigger side-effects, or may even never complete,
+ * can first check if the iterable `is EfficientLengthIterableIterable`,
+ * and if so, use [length] without those concerns.
+ *
+ * The `EfficientLengthIterable` type should never be used as an API type
+ * assertion - neither as argument type or return type of a function.
+ * Always use [Iterable] for the type and just document the performance if it
+ * is relevant. This avoids needlessly restricting the values that can be used.
+ */
+abstract class EfficientLengthIterable<T> implements Iterable<T> {
+  /**
+   * Returns the number of elements in the iterable.
+   *
+   * This is an efficient operation that doesn't iterate through
+   * the elements.
+   */
+  int get length;
+}