Produce a compile-time error if there's no main method.

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.
 
 io/raw_socket_test: Pass, RuntimeError # Issue 28288
@@ skipping 418 matching lines @@
 io/http_proxy_advanced_test: SkipByDesign
 io/http_ipv6_test: SkipByDesign
 io/socket_ipv6_test: SkipByDesign
 io/socket_info_ipv6_test: SkipByDesign
 
 [ $builder_tag == asan && $arch == x64 ]
 io/process_detached_test: Pass, Slow
 io/named_pipe_script_test: RuntimeError
 
 [ $compiler == dartk || $compiler == dartkp ]
+io/http_content_length_test: Skip # Flaky.
 io/http_proxy_test: Skip # Flaky.
 io/http_response_deadline_test: Skip # Flaky.
 io/http_reuse_server_port_test: Skip # Flaky.
 io/http_server_close_response_after_error_test: Skip # Flaky.
 io/raw_datagram_socket_test: Skip # Flaky.
+io/web_socket_test: Skip # Flaky.
 
 assert_test: RuntimeError
 io/compile_all_test: Crash
 io/http_cookie_test: RuntimeError
 io/http_proxy_advanced_test: RuntimeError
 io/socket_many_connections_test: RuntimeError
 map_insert_remove_oom_test: Crash
 package/package1_test: CompileTimeError
 package/package_test: CompileTimeError
 package/scenarios/invalid/invalid_utf8_test: CompileTimeError
@@ skipping 83 matching lines @@
 package/scenarios/packages_dir_only/packages_dir_only_test: Fail # Unable to parse package files Flutter Issue 9115
 package/scenarios/both_dir_and_file/prefers_packages_file_test: Fail # Unable to parse package files Flutter Issue 9115
 package/package_test: Fail # Unable to parse package files Flutter Issue 9115
 #unboxed_int_converter_test: Fail # Unable to parse package files Flutter Issue 9115
 io/process_run_output_test: Fail # Unable to parse package files Flutter Issue 9115
 
 io/test_harness_analyzer_test: CompileTimeError # Uses mirrors
 io/skipping_dart2js_compilations_test: CompileTimeError # Uses mirrors
 io/test_runner_test: CompileTimeError # Uses mirrors