Try to fix flakiness of test standalone/io/raw_secure_server_socket_test

tests/standalone/io/raw_secure_server_socket_test.dart

 // Copyright (c) 2013, 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=
 // VMOptions=--short_socket_read
 // VMOptions=--short_socket_write
 // VMOptions=--short_socket_read --short_socket_write
 
 import "package:expect/expect.dart";
@@ skipping 296 matching lines @@
           if (bytesRead < data.length) {
             Expect.isTrue(bytesWritten == 0);
           }
           Expect.isTrue(client.available() > 0);
           var buffer = client.read();
           if (buffer != null) {
             if (bytesRead == data.length) {
               // Read first part of TLS handshake from client.
               Expect.isTrue(postponeSecure);
               completer.complete([subscription, buffer]);
+              client.readEventsEnabled = false;
               return;
             }
             data.setRange(bytesRead, bytesRead + buffer.length, buffer);
             bytesRead += buffer.length;
             for (var value in buffer) {
               Expect.isTrue(value is int);
               Expect.isTrue(value < 256 && value >= 0);
             }
           }
           if (bytesRead == data.length) {
@@ skipping 133 matching lines @@
   testSimpleConnectFail("not_a_nickname");
   testSimpleConnectFail("CN=notARealDistinguishedName");
   testServerListenAfterConnect();
   testSimpleReadWrite(true, true, false);
   testSimpleReadWrite(true, false, false);
   testSimpleReadWrite(false, true, false);
   testSimpleReadWrite(false, false, false);
   testSimpleReadWrite(false, false, true, true);
   testSimpleReadWrite(false, false, true, false);
 }