Fix analyzer warnings in service tests

runtime/observatory/tests/service/get_heap_map_rpc_test.dart

 // Copyright (c) 2015, 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=--compile-all --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 params = {
     };
     var result = await isolate.invokeRpcNoUpgrade('getHeapMap', params);
     expect(result['type'], equals('HeapMap'));
     expect(result['freeClassId'], isPositive);
     expect(result['unitSizeBytes'], isPositive);
     expect(result['pageSizeBytes'], isPositive);
     expect(result['classList'], isNotNull);
     expect(result['pages'].length, isPositive);
     expect(result['pages'][0]['objectStart'], new isInstanceOf<String>());
     expect(result['pages'][0]['objects'].length, isPositive);
     expect(result['pages'][0]['objects'][0], isPositive);
   },
 ];
 
 main(args) async => runIsolateTests(args, tests);