Make 'where' lazy.

samples/clock/balls.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 clock;
 
 int get clientWidth => window.innerWidth;
 
 int get clientHeight => window.innerHeight;
 
@@ skipping 28 matching lines @@
     setElementSize(root, 0.0, 0.0, 0.0, 0.0);
   }
 
   void tick(num now) {
     showFps(1000.0 / (now - lastTime + 0.01));
 
     double delta = min((now - lastTime) / 1000.0, 0.1);
     lastTime = now;
 
     // incrementally move each ball, removing balls that are offscreen
-    balls = balls.where((ball) => ball.tick(delta));
+    balls = balls.where((ball) => ball.tick(delta)).toList();
     collideBalls(delta);
   }
 
   void collideBalls(double delta) {
     balls.forEach((b0) {
       balls.forEach((b1) {
         // See if the two balls are intersecting.
         double dx = (b0.x - b1.x).abs();
         double dy = (b0.y - b1.y).abs();
         double d2 = dx * dx + dy * dy;
@@ skipping 107 matching lines @@
       y = clientHeight.toDouble();
       vy *= -RESTITUTION;
     }
 
     // Position the element.
     setElementPosition(elem, x - RADIUS, y - RADIUS);
 
     return true;
   }
 }