Make 'where' 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 41ec8f38a989e81b61a84b56241dc25c10f6a496..084b328d3e7dbb7d9f1fe148a1949d864ff5a728 100644
--- a/samples/swarm/swarm_ui_lib/layout/GridLayout.dart
+++ b/samples/swarm/swarm_ui_lib/layout/GridLayout.dart
@@ -254,7 +254,7 @@ class GridLayout extends ViewLayout {
    */
   num _calcNormalizedFractionBreadth(List<GridTrack> tracks) {
 
-    final fractionTracks = tracks.where((t) => t.maxSizing.isFraction);
+    final fractionTracks = tracks.where((t) => t.maxSizing.isFraction).toList();
 
     // Note: the spec has various bugs in this function, such as mismatched
     // identifiers and names that aren't defined. For the most part it's
@@ -341,7 +341,7 @@ class GridLayout extends ViewLayout {
       ContentSizeMode sizeMode, _BreadthAccumulator breadth) {
 
     items = items.where((item) =>
-        _hasContentSizedTracks(_getTracks(item), sizeMode, breadth));
+        _hasContentSizedTracks(_getTracks(item), sizeMode, breadth)).toList();
 
     var tracks = [];