Make mappedBy lazy.

samples/swarm/swarm_ui_lib/layout/GridLayout.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.
 
 /**
  * Implements a grid-based layout system based on:
  * [http://dev.w3.org/csswg/css3-grid-align/]
  *
  * This layout is designed to support animations and work on browsers that
  * don't support grid natively. As such, we implement it on top of absolute
@@ skipping 134 matching lines @@
    * and Grid rows. The goal of the function is to ensure:
    *   1. That each Grid Track satisfies its minSizing
    *   2. That each Grid Track grows from the breadth which satisfied its
    *      minSizing to a breadth which satifies its
    *      maxSizing, subject to RemainingSpace.
    */
   // Note: spec does not correctly doc all the parameters to this function.
   void _computeUsedBreadthOfTracks(List<GridTrack> tracks) {
 
     // TODO(jmesserly): as a performance optimization we could cache this
-    final items = CollectionUtils.mappedBy(view.childViews, (view_) => view_.layout);
+    final items = view.childViews
+        .mappedBy((view_) => view_.layout)
+        .toList();
     CollectionUtils.sortBy(items, (item) => _getSpanCount(item));
 
     // 1. Initialize per Grid Track variables
     for (final t in tracks) {
       // percentage or length sizing functions will return a value
       // min-content, max-content, or a fraction will be set to 0
       t.usedBreadth = t.minSizing.resolveLength(_getGridContentSize());
       t.maxBreadth = t.maxSizing.resolveLength(_getGridContentSize());
       t.updatedBreadth = 0;
     }
@@ skipping 245 matching lines @@
   }
 
   /**
    * Scans children creating GridLayoutParams as needed, and creates all of the
    * rows and columns that we will need.
    *
    * Note: this can potentially create new rows/columns, so this needs to be
    * run before the track sizing algorithm.
    */
   void _ensureAllTracks() {
-    final items = CollectionUtils.mappedBy(view.childViews, (view_) => view_.layout);
+    final items = view.childViews.mappedBy((view_) => view_.layout);
 
     for (final child in items) {
       if (child.layoutParams == null) {
         final p = new GridLayoutParams(child.view, this);
         _ensureTrack(_rowTracks, rowSizing, p.row, p.rowSpan);
         _ensureTrack(_columnTracks, columnSizing, p.column, p.columnSpan);
         child.layoutParams = p;
       }
       child.cacheExistingBrowserLayout();
     }
   }
 
   /**
    * Given the track sizes that were computed, position children in the grid.
    */
   void _setBoundsOfChildren() {
-    final items = CollectionUtils.mappedBy(view.childViews, (view_) => view_.layout);
+    final items = view.childViews.mappedBy((view_) => view_.layout);
 
     for (final item in items) {
       GridLayoutParams childLayout = item.layoutParams;
       var xPos = _getTrackLocationX(childLayout);
       var yPos = _getTrackLocationY(childLayout);
 
       int left = xPos.start, width = xPos.length;
       int top = yPos.start, height = yPos.length;
 
       // Somewhat counterintuitively (at least to me):
@@ skipping 59 matching lines @@
     }
     return result;
   }
 
   int _getSpanCount(ViewLayout item) {
     GridLayoutParams childLayout = item.layoutParams;
     return (_dimension == Dimension.WIDTH ?
         childLayout.columnSpan : childLayout.rowSpan);
   }
 }