Fix typing for checked mode test.

runtime/bin/vmservice/observatory/test/graph_test.dart

 // Copyright (c) 2014, 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.
 
 import 'dart:async';
 import 'package:observatory/object_graph.dart';
 import 'package:observatory/service_io.dart';
 import 'package:unittest/unittest.dart';
 import 'test_helper.dart';
 
@@ skipping 17 matching lines @@
 
 var tests = [
 
 (Isolate isolate) {
   Completer completer = new Completer();
   isolate.vm.events.stream.listen((ServiceEvent event) {
     if (event.eventType == '_Graph') {
       ReadStream reader = new ReadStream(event.data);
       ObjectGraph graph = new ObjectGraph(reader);
       expect(fooId, isNotNull);
-      List<ObjectVertex> foos = graph.vertices.where((ObjectVertex obj) => obj.classId == fooId);
+      Iterable<ObjectVertex> foos = graph.vertices.where((ObjectVertex obj) => obj.classId == fooId);
       expect(foos.length, equals(3));
       expect(foos.where((ObjectVertex obj) => obj.succ.length == 0).length, equals(1));
       expect(foos.where((ObjectVertex obj) => obj.succ.length == 1).length, equals(1));
       expect(foos.where((ObjectVertex obj) => obj.succ.length == 2).length, equals(1));
       completer.complete();
     }
   });
   return isolate.rootLib.load().then((Library lib) {
     expect(lib.classes.length, equals(1));
     // Extract the numerical class id of 'Foo', used in the event listener above.
     Class fooClass = lib.classes.first;
     String prefix = "classes/";
     // TODO(koda): Add method on Class to get numerical id.
     fooId = int.parse(fooClass.id.substring(prefix.length));
     isolate.get('graph');
     return completer.future;
   });
 },
 
 ];
 
 main(args) => runIsolateTests(args, tests, testeeBefore: script);