Fix some observatory analysis issues.

runtime/observatory/tests/service/collect_all_garbage_test.dart

 // Copyright (c) 2017, 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.
 // VMOptions=--error_on_bad_type --error_on_bad_override
 
-import 'dart:async';
 import 'package:observatory/service_io.dart';
 import 'package:unittest/unittest.dart';
 
 import 'test_helper.dart';
 
 var tests = [
   (Isolate isolate) async {
     var result = await isolate.invokeRpcNoUpgrade('_collectAllGarbage', {});
     expect(result['type'], equals('Success'));
   },
 ];
 
 main(args) async => runIsolateTests(args, tests);