Mitigate #9596 for SJS: Make objects serializable only in the JVM back-end.

Author: sjrd

compiler/src/dotty/tools/backend/jvm/BCodeSkelBuilder.scala

@@ -238,11 +238,16 @@ trait BCodeSkelBuilder extends BCodeHelpers {
 
       val ps = claszSymbol.info.parents
       val superClass: String = if (ps.isEmpty) ObjectReference.internalName else internalName(ps.head.widenDealias.typeSymbol)
-      val interfaceNames = classBTypeFromSymbol(claszSymbol).info.interfaces map {
+      val interfaceNames0 = classBTypeFromSymbol(claszSymbol).info.interfaces map {
         case classBType =>
           if (classBType.isNestedClass) { innerClassBufferASM += classBType }
           classBType.internalName
       }
+      val interfaceNames =
+        if (claszSymbol.is(ModuleClass) && !interfaceNames0.contains("java/io/Serializable"))
+          interfaceNames0 :+ "java/io/Serializable"
+        else
+          interfaceNames0
 
       val flags = javaFlags(claszSymbol)
 

compiler/src/dotty/tools/dotc/ast/Desugar.scala

@@ -614,8 +614,6 @@ object desugar {
       parents1 = enumClassTypeRef :: Nil
     if (isCaseClass | isCaseObject)
       parents1 = parents1 :+ scalaDot(str.Product.toTypeName) :+ scalaDot(nme.Serializable.toTypeName)
-    else if (isObject)
-      parents1 = parents1 :+ scalaDot(nme.Serializable.toTypeName)
     if (isEnum)
       parents1 = parents1 :+ ref(defn.EnumClass.typeRef)