Dollar sign in unionKey value breaks build

[refi64]

Describe the bug

If the value of unionKey has a $, the build fails.

To Reproduce

From my application, but stripped down:

const clientToHostUnionKey = r'$stratosMessageType';

@Freezed(unionKey: clientToHostUnionKey)
abstract class ClientToHostMessage with _$ClientToHostMessage {
  factory ClientToHostMessage.requestSync(String id) = RequestSync;
  factory ClientToHostMessage.requestSyncAll() = RequestSyncAll;

  factory ClientToHostMessage.fromJson(Map<String, dynamic> json) =>
      _$ClientToHostMessageFromJson(json);
}

Expected behavior
Compilation to succeed. Instead, I get (this was from a slightly larger repro, hence why there are some branches mentioned in the error that aren't in the above example):

packages/stratos/message.freezed.dart:12:18:
Error: Getter not found: 'stratosMessageType'.
  switch (json['$stratosMessageType'] as String) {
                 ^^^^^^^^^^^^^^^^^^
packages/stratos/message.freezed.dart:228:12:
Error: The getter 'stratosMessageType' isn't defined for the class '_$LatestCaptures'.
 - '_$LatestCaptures' is from 'package:stratos/message.dart' ('packages/stratos/message.dart').
      ..['$stratosMessageType'] = 'latestCaptures';
           ^^^^^^^^^^^^^^^^^^
packages/stratos/message.freezed.dart:362:44:
Error: The getter 'stratosMessageType' isn't defined for the class '_$RequestSync'.
 - '_$RequestSync' is from 'package:stratos/message.dart' ('packages/stratos/message.dart').
    return _$_$RequestSyncToJson(this)..['$stratosMessageType'] = 'requestSync';
                                           ^^^^^^^^^^^^^^^^^^
packages/stratos/message.freezed.dart:476:12:
Error: The getter 'stratosMessageType' isn't defined for the class '_$RequestSyncAll'.
 - '_$RequestSyncAll' is from 'package:stratos/message.dart' ('packages/stratos/message.dart').
      ..['$stratosMessageType'] = 'requestSyncAll';
           ^^^^^^^^^^^^^^^^^^
Error: Compilation failed.

so it definitely seems like the unionKey value isn't being escaped properly.

Side note: this lib is pretty awesome, I've tried a bunch of Dart union libraries and this is definitely one of my favorites.