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

compiler/backend/src/org/jetbrains/kotlin/codegen/MultifileClassCodegen.kt

@@ -38,6 +38,7 @@ import org.jetbrains.kotlin.psi.*
 import org.jetbrains.kotlin.resolve.BindingContext
 import org.jetbrains.kotlin.resolve.MemberComparator
 import org.jetbrains.kotlin.resolve.jvm.JvmClassName
+import org.jetbrains.kotlin.resolve.jvm.diagnostics.JvmDeclarationOrigin
 import org.jetbrains.kotlin.resolve.jvm.diagnostics.MultifileClass
 import org.jetbrains.kotlin.resolve.jvm.diagnostics.MultifileClassPart
 import org.jetbrains.kotlin.resolve.jvm.diagnostics.OtherOrigin
@@ -50,6 +51,7 @@ import org.jetbrains.kotlin.serialization.deserialization.descriptors.Deserializ
 import org.jetbrains.org.objectweb.asm.MethodVisitor
 import org.jetbrains.org.objectweb.asm.Opcodes
 import org.jetbrains.org.objectweb.asm.Type
+import org.jetbrains.org.objectweb.asm.commons.InstructionAdapter
 import java.util.*
 
 public class MultifileClassCodegen(
@@ -86,12 +88,18 @@ public class MultifileClassCodegen(
 
         val singleSourceFile = if (previouslyCompiledCallables.isNotEmpty()) null else filesWithCallables.singleOrNull()
 
-        classBuilder.defineClass(singleSourceFile, Opcodes.V1_6, FACADE_CLASS_ATTRIBUTES,
+        classBuilder.defineClass(singleSourceFile, Opcodes.V1_6,
+                                 if (state.generateOpenMultifileClasses) OPEN_FACADE_CLASS_ATTRIBUTES else FACADE_CLASS_ATTRIBUTES,
                                  facadeClassType.internalName,
                                  null, "java/lang/Object", ArrayUtil.EMPTY_STRING_ARRAY)
         if (singleSourceFile != null) {
             classBuilder.visitSource(singleSourceFile.name, null)
         }
+
+        if (state.generateOpenMultifileClasses) {
+            generateMultifileFacadeClassConstructor(classBuilder, originFile)
+        }
+
         classBuilder
     }
 
@@ -148,6 +156,16 @@ public class MultifileClassCodegen(
         MemberCodegen.genClassOrObject(packagePartContext, classOrObject, state, null)
     }
 
+
+    private fun generateMultifileFacadeClassConstructor(classBuilder: ClassBuilder, originFile: KtFile?) {
+        val mv = classBuilder.newMethod(JvmDeclarationOrigin.NO_ORIGIN, Opcodes.ACC_PUBLIC, "<init>", "()V", null, null)
+        val v = InstructionAdapter(mv)
+        v.load(0, facadeClassType)
+        v.invokespecial("java/lang/Object", "<init>", "()V", false)
+        v.visitInsn(Opcodes.RETURN)
+        FunctionCodegen.endVisit(v, "multifile class constructor", originFile)
+    }
+
     private fun generatePart(
             file: KtFile,
             generateCallableMemberTasks: MutableMap<CallableMemberDescriptor, () -> Unit>,
@@ -280,6 +298,7 @@ public class MultifileClassCodegen(
 
     companion object {
         private val FACADE_CLASS_ATTRIBUTES = Opcodes.ACC_PUBLIC or Opcodes.ACC_FINAL
+        private val OPEN_FACADE_CLASS_ATTRIBUTES = Opcodes.ACC_PUBLIC
 
         private fun getOnlyPackageFragment(packageFqName: FqName, files: Collection<KtFile>, bindingContext: BindingContext): PackageFragmentDescriptor? {
             val fragments = SmartList<PackageFragmentDescriptor>()

compiler/backend/src/org/jetbrains/kotlin/codegen/state/GenerationState.kt

@@ -66,6 +66,7 @@ public class GenerationState @JvmOverloads constructor(
         // TODO: get rid of it with the proper module infrastructure
         public val outDirectory: File? = null,
         public val incrementalCompilationComponents: IncrementalCompilationComponents? = null,
+        public val generateOpenMultifileClasses: Boolean = false,
         public val progress: Progress = Progress.DEAF
 ) {
     public abstract class GenerateClassFilter {