Make properties in MessageKey with @JvmField to allow @JvmField in inherited classes

mirai-core-api/src/commonMain/kotlin/message/data/MessageKey.kt

@@ -1,5 +1,5 @@
 /*
- * Copyright 2019-2020 Mamoe Technologies and contributors.
+ * Copyright 2019-2021 Mamoe Technologies and contributors.
  *
  *  此源代码的使用受 GNU AFFERO GENERAL PUBLIC LICENSE version 3 许可证的约束, 可以在以下链接找到该许可证.
  *  Use of this source code is governed by the GNU AGPLv3 license that can be found through the following link.
@@ -36,6 +36,7 @@ public interface MessageKey<out M : SingleMessage> {
  * @see AbstractPolymorphicMessageKey
  */
 public abstract class AbstractMessageKey<out M : SingleMessage>(
+    @JvmField
     override val safeCast: (SingleMessage) -> M?,
 ) : MessageKey<M>
 
@@ -73,6 +74,7 @@ public abstract class AbstractPolymorphicMessageKey<out B : SingleMessage, out M
     baseKey: MessageKey<B>,
     safeCast: (SingleMessage) -> M?,
 ) : MessageKey<M>, AbstractMessageKey<M>(safeCast) {
+    @JvmField
     internal val topmostKey: MessageKey<*> =
         if (baseKey is AbstractPolymorphicMessageKey<*, *>) baseKey.topmostKey else baseKey
 }