[JS IR BE] Support String companion object

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

@@ -154,14 +154,20 @@ class JsIrBackendContext(
 
     val functions = (0..22).map { symbolTable.referenceClass(builtIns.getFunction(it)) }
 
-    val primitiveCompanionObjects = PrimitiveType.NUMBER_TYPES
-        .asSequence()
-        .filter { it.name != "LONG" && it.name != "CHAR" } // skip due to they have own explicit companions
-        .map {
-            it.typeName to symbolTable.lazyWrapper.referenceClass(
-                getClass(JS_INTERNAL_PACKAGE_FQNAME.child(Name.identifier("${it.typeName.identifier}CompanionObject")))
+    private fun primitivesWithImplicitCompanionObject(): List<Name> {
+        val numbers = PrimitiveType.NUMBER_TYPES
+            .filter { it.name != "LONG" && it.name != "CHAR" } // skip due to they have own explicit companions
+            .map { it.typeName }
+
+        return numbers + listOf(Name.identifier("String"))
+    }
+
+    val primitiveCompanionObjects = primitivesWithImplicitCompanionObject()
+        .associate {
+            it to symbolTable.lazyWrapper.referenceClass(
+                getClass(JS_INTERNAL_PACKAGE_FQNAME.child(Name.identifier("${it.identifier}CompanionObject")))
             )
-        }.toMap()
+        }
 
     val suspendFunctions = (0..22).map { symbolTable.referenceClass(builtIns.getSuspendFunction(it)) }
 

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

@@ -13,6 +13,7 @@ import org.jetbrains.kotlin.ir.expressions.IrExpression
 import org.jetbrains.kotlin.ir.expressions.IrGetObjectValue
 import org.jetbrains.kotlin.ir.expressions.impl.IrGetObjectValueImpl
 import org.jetbrains.kotlin.ir.types.isPrimitiveType
+import org.jetbrains.kotlin.ir.types.isString
 import org.jetbrains.kotlin.ir.util.defaultType
 import org.jetbrains.kotlin.ir.visitors.IrElementTransformerVoid
 import org.jetbrains.kotlin.ir.visitors.transformChildrenVoid
@@ -26,7 +27,7 @@ class PrimitiveCompanionLowering(val context: JsIrBackendContext) : FileLowering
                 val declaration = symbol.owner
                 if (!declaration.isCompanion) return expression
                 val parent = declaration.parent as IrClass
-                if (!parent.defaultType.isPrimitiveType()) return expression
+                if (!parent.defaultType.isPrimitiveType() && !parent.defaultType.isString()) return expression
                 val actualCompanion = context.primitiveCompanionObjects[parent.name] ?: return expression
                 return expression.run { IrGetObjectValueImpl(startOffset, endOffset, actualCompanion.owner.defaultType, actualCompanion) }
             }

compiler/testData/codegen/box/objectIntrinsics/objects.kt

@@ -1,4 +1,3 @@
-// IGNORE_BACKEND: JS_IR
 package foo
 
 fun box(): String {