Make 'where' lazy.

pkg/unittest/lib/mock.dart

 // Copyright (c) 2012, 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.
 
 /**
  * A simple mocking/spy library.
  *
  * To create a mock objects for some class T, create a new class using:
  *
  *     class MockT extends Mock implements T {};
@@ skipping 1443 matching lines @@
 
   /** Clear the behaviors for the Mock. */
   void resetBehavior() => _behaviors.clear();
 
   /** Clear the logs for the Mock. */
   void clearLogs() {
     if (log != null) {
       if (name == null) { // This log is not shared.
         log.logs.clear();
       } else { // This log may be shared.
-        log.logs = log.logs.where((e) => e.mockName != name);
+        log.logs = log.logs.where((e) => e.mockName != name).toList();
       }
     }
   }
 
   /** Clear both logs and behavior. */
   void reset() {
     resetBehavior();
     clearLogs();
   }
 }