Fix most UDP issues on Mac OS

tests/standalone/standalone.status

 # 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.
 
 # WARNING:
 # Tests using the multitest feature where failure is expected should *also* be
 # listed in tests/lib/analyzer/analyze_tests.status without the "standalone"
 # prefix.
 
 package/invalid_uri_test: Fail, OK # CompileTimeErrors intentionally
@@ skipping 13 matching lines @@
 # These test have type errors on purpose and take very long to run in
 # checked mode with no benefit. Skip.
 io/file_fuzz_test: Skip
 io/directory_fuzz_test: Skip
 
 [ $runtime == vm && $system == macos ]
 # This test fails with "Too many open files" on the Mac OS buildbot.
 # This is expected as MacOS by default runs with a very low number
 # of allowed open files ('ulimit -n' says something like 256).
 io/socket_many_connections_test: Skip
-# The UDP tests are currently skipped on Mac OS as multicast configuration
-# needs to be resolved.
-io/raw_datagram_socket_test: Skip # Issue 1975
 
 [ $compiler == none && ($runtime == drt || $runtime == dartium) ]
 typed_array_test: Fail # Issue 13921
 typed_array_int64_uint64_test: Fail # Issue 13921
 typed_data_isolate_test: Skip # This test uses dart:io
 io/*: Skip # Don't run tests using dart:io in the browser
 package/*: Skip # Do not run those in Dartium.
 debugger/*: Skip # Do not run standalone debugger tests in browser.
 coverage_test: Skip
 http_launch_test: Skip
@@ skipping 86 matching lines @@
 io/directory_invalid_arguments_test: StaticWarning
 io/process_invalid_arguments_test: StaticWarning
 io/raw_secure_server_socket_argument_test: StaticWarning
 io/secure_socket_argument_test: StaticWarning
 io/stdout_bad_argument_test: StaticWarning
 package/package1_test: StaticWarning
 package/package_test: StaticWarning
 typed_data_test: StaticWarning
 typed_data_view_test: StaticWarning