Fix analyzer error in get_ports_rpc_test.dart

runtime/observatory/tests/service/get_ports_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
 
 library get_ports_rpc_test;
 
 import 'dart:async';
-import 'dart:isolate';
+import 'dart:isolate' hide Isolate;
 import 'package:observatory/service_io.dart';
 import 'package:unittest/unittest.dart';
 
 import 'test_helper.dart';
 
 var port1;
 var port2;
 
 void warmup() {
   port1 = new RawReceivePort(null);
@@ skipping 27 matching lines @@
     var ports = result['ports'];
     // There are three ports: the two created in warmup and the stdin listener
     // created by the test harness.
     expect(ports.length, equals(3));
     expect(countHandlerMatches(ports, nullMatcher), equals(1));
     expect(countHandlerMatches(ports, closureMatcher), equals(2));
   },
 ];
 
 main(args) async => runIsolateTests(args, tests, testeeBefore:warmup);