Add EfficientLength marker interface to some iterabels.

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 67 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);
     int count = 0;
     Iterator it = iterator;
     while (it.moveNext()) {
       count++;
     }
     return count;
   }
 
   bool get isEmpty => !iterator.moveNext();
 
@@ skipping 171 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);
     int count = 0;
     Iterator it = iterator;
     while (it.moveNext()) {
       count++;
     }
     return count;
   }
 
   bool get isEmpty => !iterator.moveNext();
 
@@ skipping 84 matching lines @@
   E elementAt(int index) {
     if (index is! int || index < 0) throw new RangeError.value(index);
     int remaining = index;
     for (E element in this) {
       if (remaining == 0) return element;
       remaining--;
     }
     throw new RangeError.value(index);
   }
 }