Make 'where' lazy.

sdk/lib/core/sequences.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.
 
 /**
  * An indexed sequence of elements of the same type.
  *
  * This is a primitive interface that any finite integer-indexable
  * sequence can implement.
  * It is intended for data structures where access by index is
@@ skipping 38 matching lines @@
   }
 
   reduce(initialValue, combine(previousValue, E element)) {
     var value = initialValue;
     for (int i = 0; i < this.length; i++) {
       value = combine(value, this[i]);
     }
     return value;
   }
 
-  Collection<E> where(bool f(E element)) {
-    List<E> result = <E>[];
-    for (int i = 0; i < this.length; i++) {
-      E element = this[i];
-      if (f(element)) result.add(element);
-    }
-    return result;
-  }
+  Iterable<E> where(bool f(E element)) => new WhereIterable<E>(this, f);
 
   bool every(bool f(E element)) {
     for (int i = 0; i < this.length; i++) {
       if (!f(this[i])) return false;
     }
     return true;
   }
 
   bool some(bool f(E element)) {
     for (int i = 0; i < this.length; i++) {
@@ skipping 128 matching lines @@
   }
   E get current {
     if (0 <= _position && _position < _sequence.length) {
       return _sequence[_position];
     }
     // TODO(floitsch): adapt error-message.
     throw new StateError("No more elements");
   }
 }