Remove Collection, Collections and clean up List/Set/Queue implementations of retain/remove.

samples/swarm/swarm_ui_lib/layout/ViewLayout.dart

 // Copyright (c) 2011, 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 layout;
 
 /** The interface that the layout algorithms use to talk to the view. */
 abstract class Positionable {
   ViewLayout get layout;
 
   /** Gets our custom CSS properties, as provided by the CSS preprocessor. */
   Map<String, String> get customStyle;
 
   /** Gets the root DOM used for layout. */
   Element get node;
 
   /** Gets the collection of child views. */
-  Collection<Positionable> get childViews;
+  Iterable<Positionable> get childViews;
 
   /** Causes a view to layout its children. */
   void doLayout();
 }
 
 
 /**
  * Caches the layout parameters that were specified in CSS during a layout
  * computation. These values are immutable during a layout.
  */
@@ skipping 198 matching lines @@
       // For an unset max-content size, use the actual size
       return size;
     }
     if (style.endsWith('%')) {
       num percent = double.parse(style.substring(0, style.length - 1));
       return ((percent / 100) * parentSize).toInt();
     }
     return _toPixels(style);
   }
 }