[JS IR] add a flag which enable safe property access

If an accessor is not available at runtime we fall back
to the property access.

This is useful in cases when JS objects are casted to Kotlin
classes implicitly. This pattern did work in the old BE, which
lead to a significant amount of code which doesn't work anymore.

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

@@ -94,6 +94,11 @@ private fun buildRoots(modules: Iterable<IrModuleFragment>, context: JsIrBackend
         rootDeclarations += dceRuntimeDiagnostic.unreachableDeclarationMethod(context).owner
     }
 
+    if (context.legacyPropertyAccess) {
+        rootDeclarations += context.intrinsics.safePropertyGet.owner
+        rootDeclarations += context.intrinsics.safePropertySet.owner
+    }
+
     JsMainFunctionDetector.getMainFunctionOrNull(modules.last())?.let { mainFunction ->
         rootDeclarations += mainFunction
         if (mainFunction.isSuspend) {

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

@@ -315,6 +315,10 @@ class JsIntrinsics(private val irBuiltIns: IrBuiltIns, val context: JsIrBackendC
     val readSharedBox = defineReadSharedBox()
     val writeSharedBox = defineWriteSharedBox()
 
+
+    val safePropertyGet = getInternalFunction("safePropertyGet")
+    val safePropertySet = getInternalFunction("safePropertySet")
+
     val jsUndefined = defineJsUndefinedIntrinsic()
 
     // Helpers:

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

@@ -47,6 +47,7 @@ class JsIrBackendContext(
     override val es6mode: Boolean = false,
     val dceRuntimeDiagnostic: DceRuntimeDiagnostic? = null,
     val propertyLazyInitialization: Boolean = false,
+    val legacyPropertyAccess: Boolean = false,
 ) : JsCommonBackendContext {
     val fileToInitializationFuns: MutableMap<IrFile, IrSimpleFunction?> = mutableMapOf()
     val fileToInitializerPureness: MutableMap<IrFile, Boolean> = mutableMapOf()

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

@@ -52,6 +52,7 @@ fun compile(
     multiModule: Boolean = false,
     relativeRequirePath: Boolean = false,
     propertyLazyInitialization: Boolean,
+    legacyPropertyAccess: Boolean = false,
 ): CompilerResult {
     val (moduleFragment: IrModuleFragment, dependencyModules, irBuiltIns, symbolTable, deserializer) =
         loadIr(project, mainModule, analyzer, configuration, allDependencies, friendDependencies, irFactory)
@@ -73,6 +74,7 @@ fun compile(
         es6mode = es6mode,
         dceRuntimeDiagnostic = dceRuntimeDiagnostic,
         propertyLazyInitialization = propertyLazyInitialization,
+        legacyPropertyAccess = legacyPropertyAccess
     )
 
     // Load declarations referenced during `context` initialization

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

@@ -110,7 +110,7 @@ fun translateCall(
     // @JsName-annotated external property accessors are translated as function calls
     if (function.getJsName() == null) {
         val property = function.correspondingPropertySymbol?.owner
-        if (property != null && property.isEffectivelyExternal()) {
+        if (property != null && (property.isEffectivelyExternal())) {
             val nameRef = JsNameRef(context.getNameForProperty(property), jsDispatchReceiver)
             return when (function) {
                 property.getter -> nameRef
@@ -220,7 +220,26 @@ fun translateCall(
             }
         }
     } else {
-        JsInvocation(ref, listOfNotNull(jsExtensionReceiver) + arguments)
+        val defaultResult = JsInvocation(ref, listOfNotNull(jsExtensionReceiver) + arguments)
+
+        val alternativeResult = if (jsDispatchReceiver != null && jsExtensionReceiver == null && context.staticContext.backendContext.legacyPropertyAccess) {
+            val property = function.correspondingPropertySymbol?.owner
+            if (property != null) {
+                val propertyName = context.getNameForProperty(property)
+                val args = mutableListOf(jsDispatchReceiver, JsStringLiteral(symbolName.ident), JsStringLiteral(propertyName.ident))
+                val fnName = when (function) {
+                    property.getter -> context.getNameForStaticFunction(context.staticContext.backendContext.intrinsics.safePropertyGet.owner)
+                    property.setter -> {
+                        args += arguments
+                        context.getNameForStaticFunction(context.staticContext.backendContext.intrinsics.safePropertySet.owner)
+                    }
+                    else -> error("Function must be an accessor of corresponding property")
+                }
+                JsInvocation(fnName.makeRef(), args)
+            } else null
+        } else null
+
+        alternativeResult ?: defaultResult
     }
 }