Make mappedBy 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 243 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);
-  Collection mappedBy(f(T element)) => _internal.mappedBy(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
 // much. Also, making a value observable necessitates adding ".value" to lots
@@ skipping 10 matching lines @@
     // Only fire on an actual change.
     if (!identical(newValue, _value)) {
       final oldValue = _value;
       _value = newValue;
       recordPropertyUpdate("value", newValue, oldValue);
     }
   }
 
   T _value;
 }