fix observe package to be strong mode clean

test/observe_test_utils.dart

 // Copyright (c) 2013, 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 observe.test.observe_test_utils;
 
 import 'dart:async';
 import 'package:observe/observe.dart';
 import 'package:observe/mirrors_used.dart' as mu; // to make tests smaller
 import 'package:observe/src/dirty_check.dart';
@@ skipping 24 matching lines @@
 ///     future.then(newMicrotask).then(...)
 ///
 /// Uses [mu].
 newMicrotask(_) => new Future.value();
 
 // TODO(jmesserly): use matchers when we have a way to compare ChangeRecords.
 // For now just use the toString.
 expectChanges(actual, expected, {reason}) =>
     expect('$actual', '$expected', reason: reason);
 
-List getListChangeRecords(List changes, int index) => changes
-    .where((c) => c.indexChanged(index)).toList();
+List<ListChangeRecord> getListChangeRecords(List changes, int index) =>
+    new List.from(changes.where((c) => c.indexChanged(index)));
 
-List getPropertyChangeRecords(List changes, Symbol property) => changes
-    .where((c) => c is PropertyChangeRecord && c.name == property).toList();
+List<PropertyChangeRecord> getPropertyChangeRecords(
+        List changes, Symbol property) =>
+    new List.from(
+        changes.where((c) => c is PropertyChangeRecord && c.name == property));