For migration: Make an option "Xmultifile-facades-open" for compiler to generate open multifile facade classes.

compiler/cli/src/org/jetbrains/kotlin/cli/jvm/K2JVMCompiler.kt

@@ -275,6 +275,7 @@ public open class K2JVMCompiler : CLICompiler<K2JVMCompilerArguments>() {
             configuration.put(JVMConfigurationKeys.DISABLE_PARAM_ASSERTIONS, arguments.noParamAssertions)
             configuration.put(JVMConfigurationKeys.DISABLE_INLINE, arguments.noInline)
             configuration.put(JVMConfigurationKeys.DISABLE_OPTIMIZATION, arguments.noOptimize)
+            configuration.put(JVMConfigurationKeys.MULTIFILE_FACADES_OPEN, arguments.multifileFacadesOpen);
         }
 
         private fun getClasspath(paths: KotlinPaths, arguments: K2JVMCompilerArguments): List<File> {

compiler/cli/src/org/jetbrains/kotlin/cli/jvm/compiler/KotlinToJVMBytecodeCompiler.java

@@ -438,7 +438,8 @@ public class KotlinToJVMBytecodeCompiler {
                 targetId,
                 moduleName,
                 outputDirectory,
-                incrementalCompilationComponents
+                incrementalCompilationComponents,
+                configuration.get(JVMConfigurationKeys.MULTIFILE_FACADES_OPEN, false)
         );
         ProgressIndicatorAndCompilationCanceledStatus.checkCanceled();
 

compiler/cli/src/org/jetbrains/kotlin/cli/jvm/config/JVMConfigurationKeys.java

@@ -35,6 +35,8 @@ public class JVMConfigurationKeys {
             CompilerConfigurationKey.create("disable inline");
     public static final CompilerConfigurationKey<Boolean> DISABLE_OPTIMIZATION =
             CompilerConfigurationKey.create("disable optimization");
+    public static final CompilerConfigurationKey<Boolean> MULTIFILE_FACADES_OPEN =
+            CompilerConfigurationKey.create("compile multifile facade classes as open");
 
     public static final CompilerConfigurationKey<IncrementalCompilationComponents> INCREMENTAL_COMPILATION_COMPONENTS =
             CompilerConfigurationKey.create("incremental cache provider");