Change DCE a bit to touch less declarations

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

@@ -78,16 +78,9 @@ private fun removeUselessDeclarations(module: IrModuleFragment, usefulDeclaratio
                 process(declaration)
             }
 
-            override fun visitConstructor(declaration: IrConstructor) {
-                if (declaration !in usefulDeclarations) {
-                    // Keep the constructor declaration without body in order to declare the JS constructor function
-                    declaration.body = IrBlockBodyImpl(UNDEFINED_OFFSET, UNDEFINED_OFFSET, emptyList())
-                }
-            }
-
             private fun process(container: IrDeclarationContainer) {
                 container.declarations.transformFlat { member ->
-                    if (member !in usefulDeclarations && member !is IrConstructor) {
+                    if (member !in usefulDeclarations) {
                         emptyList()
                     } else {
                         member.acceptVoid(this)

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

@@ -33,6 +33,7 @@ class JsClassGenerator(private val irClass: IrClass, val context: JsGenerationCo
     fun generate(): JsStatement {
         assert(!irClass.descriptor.isExpect)
 
+        maybeGeneratePrimaryConstructor()
         val transformer = IrDeclarationToJsTransformer()
 
         // Properties might be lowered out of classes
@@ -142,6 +143,15 @@ class JsClassGenerator(private val irClass: IrClass, val context: JsGenerationCo
         return null
     }
 
+    private fun maybeGeneratePrimaryConstructor() {
+        if (!irClass.declarations.any { it is IrConstructor }) {
+            val func = JsFunction(emptyScope, JsBlock(), "Ctor for ${irClass.name}")
+            func.name = className
+            classBlock.statements += func.makeStmt()
+            classModel.preDeclarationBlock.statements += generateInheritanceCode()
+        }
+    }
+
     private fun generateInheritanceCode(): List<JsStatement> {
         if (baseClass == null || baseClass.isAny()) {
             return emptyList()