Address static warning about service library's Isolate hiding dart:isolate's Isolate.

runtime/observatory/tests/service/pause_idle_isolate_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=--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:io';
-import 'dart:isolate';
+import 'dart:isolate' show ReceivePort;
 
 var receivePort;
 
 void testMain() {
   receivePort = new ReceivePort();
 }
 
 var tests = [
 
 (Isolate isolate) async {
@@ skipping 49 matching lines @@
   expect(receivedInterrupt, isTrue);
 },
 
 ];
 
 main(args) => runIsolateTests(args, tests,
                               testeeConcurrent: testMain,
                               pause_on_start: true,
                               trace_service: true,
                               verbose_vm: true);