[JS IR] Use a backing field initializer for js() code folding

During a translation of the code from js() call to statement list,
 the compiler folds the code string. For that it uses property bodies.
 However the bodies can not be loaded in an incremental rebuild.
 The patch uses the backing field initializers of constant properties.
 The initializers are always loaded.

^KT-57002 Fixed

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

@@ -13,6 +13,7 @@ import org.jetbrains.kotlin.ir.backend.js.transformers.irToJs.CrossModuleReferen
 import org.jetbrains.kotlin.ir.IrElement
 import org.jetbrains.kotlin.ir.declarations.IrAnnotationContainer
 import org.jetbrains.kotlin.ir.declarations.IrFunction
+import org.jetbrains.kotlin.ir.declarations.IrProperty
 import org.jetbrains.kotlin.ir.declarations.IrTypeParametersContainer
 import org.jetbrains.kotlin.ir.symbols.IrSymbol
 import org.jetbrains.kotlin.ir.util.DumpIrTreeVisitor
@@ -131,6 +132,11 @@ internal class ICHasher {
             }
         }
         (symbol.owner as? IrAnnotationContainer)?.let(hashCalculator::updateAnnotationContainer)
+        (symbol.owner as? IrProperty)?.let { irProperty ->
+            if (irProperty.isConst) {
+                irProperty.backingField?.initializer?.let(hashCalculator::update)
+            }
+        }
         return hashCalculator.finalize()
     }
 }

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

@@ -63,7 +63,7 @@ private fun translateJsCodeIntoStatementList(
     return parseExpressionOrStatement(jsCode, ThrowExceptionOnErrorReporter, currentScope, CodePosition(startLine, offset), fileName)
 }
 
-fun foldString(expression: IrExpression, context: JsIrBackendContext?): String? {
+private fun foldString(expression: IrExpression, context: JsIrBackendContext?): String? {
     val builder = StringBuilder()
     var foldingFailed = false
     expression.acceptVoid(object : IrElementVisitorVoid {
@@ -92,6 +92,7 @@ fun foldString(expression: IrExpression, context: JsIrBackendContext?): String?
 
         override fun visitCall(expression: IrCall) {
             val owner = expression.symbol.owner
+            val propertySymbol = owner.correspondingPropertySymbol
             return when {
                 expression.origin == IrStatementOrigin.PLUS ->
                     expression.acceptChildrenVoid(this)
@@ -99,8 +100,17 @@ fun foldString(expression: IrExpression, context: JsIrBackendContext?): String?
                     owner.body?.acceptChildrenVoid(InitFunVisitor(context))
                     expression.acceptChildrenVoid(this)
                 }
-                owner == owner.correspondingPropertySymbol?.owner?.getter -> {
-                    owner.body?.acceptChildrenVoid(this)
+                propertySymbol != null && owner == propertySymbol.owner.getter -> {
+                    if (propertySymbol.owner.isConst) {
+                        val initializer = propertySymbol.owner.backingField?.initializer
+                        if (initializer != null) {
+                            initializer.acceptChildrenVoid(this)
+                        } else {
+                            foldingFailed = true
+                        }
+                    } else {
+                        owner.body?.acceptChildrenVoid(this)
+                    }
                     expression.acceptChildrenVoid(this)
                 }
                 else -> super.visitCall(expression)