Introduce ClassDescriptor.getSealedSubclasses

#KT-12795 In Progress

compiler/backend/src/org/jetbrains/kotlin/codegen/MutableClassDescriptor.java

@@ -33,7 +33,7 @@ import org.jetbrains.kotlin.types.TypeUtils;
 
 import java.util.*;
 
-public class MutableClassDescriptor extends ClassDescriptorBase implements ClassDescriptor {
+public class MutableClassDescriptor extends ClassDescriptorBase {
     private final ClassKind kind;
     private final boolean isInner;
 
@@ -71,6 +71,7 @@ public class MutableClassDescriptor extends ClassDescriptorBase implements Class
     }
 
     public void setModality(@NotNull Modality modality) {
+        assert modality != Modality.SEALED : "Implement getSealedSubclasses() for this class: " + getClass();
         this.modality = modality;
     }
 
@@ -180,6 +181,12 @@ public class MutableClassDescriptor extends ClassDescriptorBase implements Class
         return MemberScope.Empty.INSTANCE;
     }
 
+    @NotNull
+    @Override
+    public Collection<ClassDescriptor> getSealedSubclasses() {
+        return Collections.emptyList();
+    }
+
     @Override
     public String toString() {
         return DeclarationDescriptorImpl.toString(this);