Make 'where' lazy.

samples/swarm/swarm_ui_lib/observable/observable.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.
 
 library observable;
 
 part 'ChangeEvent.dart';
 part 'EventBatch.dart';
 
 /**
@@ skipping 57 matching lines @@
       return true;
     }
 
     return false;
   }
 
   bool removeChangeListener(ChangeListener listener) {
     // 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;
     }
   }
 
   void recordPropertyUpdate(String propertyName, newValue, oldValue) {
     recordEvent(new ChangeEvent.property(
         this, propertyName, newValue, oldValue));
   }
@@ skipping 105 matching lines @@
 
   T removeAt(int index) {
     int i = 0;
     T found = null;
     _internal = _internal.where(bool _(element) {
       if (i++ == index) {
         found = element;
         return false;
       }
       return true;
-    });
+    }).toList();
     if (found != null) {
       recordListRemove(index, found);
     }
     return found;
   }
 
   int indexOf(T element, [int start = 0]) {
     return _internal.indexOf(element, start);
   }
 
@@ skipping 38 matching lines @@
   }
 
   List getRange(int start, int length) {
     throw new UnimplementedError();
   }
 
   // Iterable<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);
   void forEach(void f(T element)) { _internal.forEach(f); }
   bool get isEmpty => length == 0;
 }
 
 // TODO(jmesserly): is this too granular? Other similar systems make whole
 // classes observable instead of individual fields. The memory cost of having
 // every field effectively boxed, plus having a listeners list is likely too
@@ skipping 11 matching lines @@
     // Only fire on an actual change.
     if (!identical(newValue, _value)) {
       final oldValue = _value;
       _value = newValue;
       recordPropertyUpdate("value", newValue, oldValue);
     }
   }
 
   T _value;
 }