title: I migrated to null safety and i cant run "flutter pub run build_runner build" without an error being thrown
body: <p>I have added <code>// @dart=2.9</code> to all my files but the <code>build_runner</code> won't do its thing without throwing this error:</p>
  <pre><code>Warning: Operand of null-aware operation '?.' has type 'SendPort' which excludes null. - 'SendPort' is from 'dart:isolate'.  sendPort?.send(result);  ^Error: Cannot run with sound null safety, because the following dependencies don't support null safety: 
  - package:build_runner_core 
  - package:json_serializable 
  - package:moor_generator 
  - package:objectbox_generator 
  - package:retrofit_generator 
  - package:source_gen 
  - package:build_config 
  - package:build_runner 
  - package:build 
  - package:json_annotation 
  - package:glob 
  - package:pool 
  - package:crypto 
  - package:logging 
  - package:watcher 
  - package:build_resolvers 
  - package:timing 
  - package:graphs 
  - package:package_config 
  - package:yaml 
  - package:analyzer 
  - package:sqlparser 
  - package:objectbox 
  - package:dart_style 
  - package:built_collection 
  - package:code_builder 
  - package:dio 
  - package:tuple 
  - package:retrofit 
  - package:pedantic 
  - package:checked_yaml 
  - package:pubspec_parse 
  - package:build_daemon 
  - package:args 
  - package:io 
  - package:convert 
  - package:pub_semver 
  - package:ffi 
  - package:quiver 
  - package:moor 
  - package:recase 
  - package:_fe_analyzer_shared 
  - package:built_value 
  - package:http_parser 
  - package:http_multi_server 
  - package:shelf 
  - package:stream_transform 
  - package:mime 
  - package:shelf_web_socket 
  - package:web_socket_channel 
  - package:cli_util 
  - package:fixnum 
  - package:synchronizedFor solutions, see https://dart.dev/go/unsound-null-safety.dart_tool/build/entrypoint/build.dart:44:44: Error: The parameter 'sendPort' can't have a value of 'null' because of its type 'SendPort', but the implicit default value is 'null'. - 'SendPort' is from 'dart:isolate'.Try adding either an explicit non-'null' default value or the 'required' modifier.void main(List&lt;String&gt; args, [_i8.SendPort sendPort]) async { 
  </code></pre>