Fix analyzer warnings in service tests

runtime/observatory/tests/service/get_stack_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 'package:observatory/service_io.dart';
 import 'package:unittest/unittest.dart';
 import 'test_helper.dart';
 import 'dart:async';
-import 'dart:isolate';
+import 'dart:isolate' as isolate;
 
 int counter = 0;
 const stoppedAtLine = 23;
-var port = new RawReceivePort(msgHandler);
+var port = new isolate.RawReceivePort(msgHandler);
 
 // This name is used in a test below.
 void msgHandler(_) {
 }
 
 void periodicTask(_) {
   counter++;
   port.sendPort.send(34);
   counter++;  // Line 23.  We set our breakpoint here.
   counter++;
@@ skipping 84 matching lines @@
   expect(msgHandlerObjectId, isNotNull);
 
   // Get object.
   var object = await isolate.getObject(msgHandlerObjectId);
   expect(object.valueAsString, equals('34'));
 }
 
 ];
 
 main(args) => runIsolateTests(args, tests, testeeBefore: startTimer);