[IR] minor: make StageController an open class

This should make creating a new "do nothing" controller more straightforward.

compiler/ir/backend.js/src/org/jetbrains/kotlin/ir/backend/js/MutableController.kt

@@ -13,7 +13,7 @@ import org.jetbrains.kotlin.ir.declarations.impl.IrDeclarationBase
 import org.jetbrains.kotlin.ir.expressions.IrBody
 import org.jetbrains.kotlin.ir.util.isLocal
 
-open class MutableController(val context: JsIrBackendContext, val lowerings: List<Lowering>) : StageController {
+open class MutableController(val context: JsIrBackendContext, val lowerings: List<Lowering>) : StageController() {
 
     override var currentStage: Int = 0
 

compiler/ir/backend.js/src/org/jetbrains/kotlin/ir/backend/js/compiler.kt

@@ -49,7 +49,7 @@ fun compile(
     multiModule: Boolean = false,
     relativeRequirePath: Boolean = false
 ): CompilerResult {
-    stageController = object : StageController {}
+    stageController = StageController()
 
     val (moduleFragment: IrModuleFragment, dependencyModules, irBuiltIns, symbolTable, deserializer) =
         loadIr(project, mainModule, analyzer, configuration, allDependencies, friendDependencies)
@@ -86,9 +86,7 @@ fun compile(
         eliminateDeadDeclarations(allModules, context)
 
         // TODO investigate whether this is needed anymore
-        stageController = object : StageController {
-            override val currentStage: Int = controller.currentStage
-        }
+        stageController = StageController(controller.currentStage)
 
         val transformer = IrModuleToJsTransformer(
             context,

compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/declarations/PersistentApi.kt

@@ -10,31 +10,29 @@ import org.jetbrains.kotlin.ir.expressions.IrBody
 
 // TODO threadlocal
 // TODO make a IrDeclarationBase field? (requires IR factory)
-var stageController: StageController = object : StageController {}
+var stageController: StageController = StageController()
 
 // TODO make a class
-interface StageController {
-    val currentStage: Int get() = 0
+open class StageController(open val currentStage: Int = 0) {
+    open fun lazyLower(declaration: IrDeclaration) {}
 
-    fun lazyLower(declaration: IrDeclaration) {}
+    open fun lazyLower(body: IrBody) {}
 
-    fun lazyLower(body: IrBody) {}
+    open fun <T> withStage(stage: Int, fn: () -> T): T = fn()
 
-    fun <T> withStage(stage: Int, fn: () -> T): T = fn()
+    open val bodiesEnabled: Boolean get() = true
 
-    val bodiesEnabled: Boolean get() = true
+    open fun <T> withInitialIr(block: () -> T): T = block()
 
-    fun <T> withInitialIr(block: () -> T): T = block()
+    open fun <T> restrictTo(declaration: IrDeclaration, fn: () -> T): T = fn()
 
-    fun <T> restrictTo(declaration: IrDeclaration, fn: () -> T): T = fn()
+    open fun <T> bodyLowering(fn: () -> T): T = fn()
 
-    fun <T> bodyLowering(fn: () -> T): T = fn()
+    open fun canModify(element: IrElement): Boolean = true
 
-    fun canModify(element: IrElement): Boolean = true
+    open fun <T> unrestrictDeclarationListsAccess(fn: () -> T): T = fn()
 
-    fun <T> unrestrictDeclarationListsAccess(fn: () -> T): T = fn()
-
-    fun canAccessDeclarationsOf(irClass: IrClass): Boolean = true
+    open fun canAccessDeclarationsOf(irClass: IrClass): Boolean = true
 }
 
 @Suppress("NOTHING_TO_INLINE")