Make mappedBy lazy.

samples/swarm/swarm_ui_lib/layout/GridLayout.dart

Index: samples/swarm/swarm_ui_lib/layout/GridLayout.dart
diff --git a/samples/swarm/swarm_ui_lib/layout/GridLayout.dart b/samples/swarm/swarm_ui_lib/layout/GridLayout.dart
index e20d2d8138d0c0c4090ca8781b86ab786b8ac7ab..41ec8f38a989e81b61a84b56241dc25c10f6a496 100644
--- a/samples/swarm/swarm_ui_lib/layout/GridLayout.dart
+++ b/samples/swarm/swarm_ui_lib/layout/GridLayout.dart
@@ -152,7 +152,9 @@ class GridLayout extends ViewLayout {
   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
@@ -418,7 +420,7 @@ class GridLayout extends ViewLayout {
    * 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) {
@@ -435,7 +437,7 @@ class GridLayout extends ViewLayout {
    * 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;