Fix get_ports_rpc_test to be insensitive to system level ports

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=--error_on_bad_type --error_on_bad_override
 
 library get_ports_rpc_test;
 
 import 'dart:isolate' hide Isolate;
 import 'package:observatory/service_io.dart';
 import 'package:unittest/unittest.dart';
@@ skipping 26 matching lines @@
 bool closureMatcher(handler) {
   return handler.isClosure;
 }
 
 var tests = [
   (Isolate isolate) async {
     var result = await isolate.invokeRpc('_getPorts', {});
     expect(result['type'], equals('_Ports'));
     expect(result['ports'], isList);
     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));
+    // There are at least two ports: the two created in warm up. Some OSes
+    // will have other ports open but we do not try and test for these.
+    expect(ports.length, greaterThanOrEqualTo(2));
+    expect(countHandlerMatches(ports, nullMatcher), greaterThanOrEqualTo(1));
+    expect(countHandlerMatches(ports, closureMatcher), greaterThanOrEqualTo(1));
   },
 ];
 
 main(args) async => runIsolateTests(args, tests, testeeBefore:warmup);