Make 'where' lazy.

samples/swarm/swarm_ui_lib/observable/observable.dart

Index: samples/swarm/swarm_ui_lib/observable/observable.dart
diff --git a/samples/swarm/swarm_ui_lib/observable/observable.dart b/samples/swarm/swarm_ui_lib/observable/observable.dart
index b702a3dc9599ab09ab99133dae30ccfc00ea88c1..6318de7ea571dd814edbb8804f4264028d5a0b30 100644
--- a/samples/swarm/swarm_ui_lib/observable/observable.dart
+++ b/samples/swarm/swarm_ui_lib/observable/observable.dart
@@ -75,7 +75,9 @@ class AbstractObservable implements Observable {
     // TODO(rnystrom): This is awkward without List.remove(e).
     if (listeners.indexOf(listener, 0) != -1) {
       bool found = false;
-      listeners = listeners.where((e) => found || !(found = (e == listener)));
+      listeners = listeners
+          .where((e) => found || !(found = (e == listener)))
+          .toList();
       return true;
     } else {
       return false;
@@ -201,7 +203,7 @@ class ObservableList<T>
         return false;
       }
       return true;
-    });
+    }).toList();
     if (found != null) {
       recordListRemove(index, found);
     }
@@ -260,7 +262,7 @@ class ObservableList<T>
   Iterator<T> iterator() => _internal.iterator();
 
   // Collection<T>:
-  Collection<T> where(bool f(T element)) => _internal.where(f);
+  Iterable<T> where(bool f(T element)) => _internal.where(f);
   Iterable mappedBy(f(T element)) => _internal.mappedBy(f);
   bool every(bool f(T element)) => _internal.every(f);
   bool some(bool f(T element)) => _internal.some(f);