[K/JS] fix(KT-46643): prevent removal of setter for overridden external field.

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

@@ -77,7 +77,8 @@ private fun IrDeclaration.addRootsTo(
             }
         }
         this is IrSimpleFunction -> {
-            if (correspondingPropertySymbol?.owner?.isExported(context) == true) {
+            val correspondingProperty = correspondingPropertySymbol?.owner ?: return
+            if (correspondingProperty.isExported(context)) {
                 acceptVoid(nestedVisitor)
             }
         }

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

@@ -185,6 +185,10 @@ abstract class UsefulDeclarationProcessor(
             }
         }
 
+        if (declaration is IrSimpleFunction && declaration.isAccessorForOverriddenExternalField()) {
+            declaration.enqueue(declaration.correspondingPropertySymbol!!.owner, "overrides external property")
+        }
+
         // A hack to enforce property lowering.
         // Until a getter is accessed it doesn't get moved to the declaration list.
         if (declaration is IrProperty) {
@@ -197,6 +201,14 @@ abstract class UsefulDeclarationProcessor(
         }
     }
 
+    private fun IrSimpleFunction.isAccessorForOverriddenExternalField(): Boolean {
+        return correspondingPropertySymbol?.owner?.isExternalOrOverriddenExternal() ?: false
+    }
+
+    private fun IrProperty.isExternalOrOverriddenExternal(): Boolean {
+        return isEffectivelyExternal() || overriddenSymbols.any { it.owner.isExternalOrOverriddenExternal() }
+    }
+
     protected open fun handleAssociatedObjects(): Unit = Unit
 
     fun collectDeclarations(rootDeclarations: Iterable<IrDeclaration>): Set<IrDeclaration> {

compiler/ir/backend.js/src/org/jetbrains/kotlin/ir/backend/js/transformers/irToJs/JsClassGenerator.kt

@@ -416,7 +416,7 @@ fun IrSimpleFunction?.shouldExportAccessor(context: JsIrBackendContext): Boolean
 
     if (parentAsClass.isExported(context)) return true
 
-    return overriddenStableProperty(context)
+    return isAccessorOfOverriddenStableProperty(context)
 }
 
 fun IrSimpleFunction.overriddenStableProperty(context: JsIrBackendContext): Boolean {
@@ -429,10 +429,14 @@ fun IrSimpleFunction.overriddenStableProperty(context: JsIrBackendContext): Bool
     return overridesExternal() || property.getJsName() != null
 }
 
-private fun IrSimpleFunction.overridesExternal(): Boolean {
+fun IrSimpleFunction.isAccessorOfOverriddenStableProperty(context: JsIrBackendContext): Boolean {
+    return overriddenStableProperty(context) || correspondingPropertySymbol!!.owner.overridesExternal()
+}
+
+private fun IrOverridableDeclaration<*>.overridesExternal(): Boolean {
     if (this.isEffectivelyExternal()) return true
 
-    return this.overriddenSymbols.any { it.owner.overridesExternal() }
+    return overriddenSymbols.any { (it.owner as IrOverridableDeclaration<*>).overridesExternal() }
 }
 
 private val IrClassifierSymbol.isInterface get() = (owner as? IrClass)?.isInterface == true