[FIR2IR] Fix NPE on missing built-ins when using old stdlib

Specifically, this code was throwing when trying to get the symbol for
IntrinsicConstEvaluation which is only available since 1.7.

#KT-59151 Fixed

Author: cypressious

compiler/fir/fir2ir/src/org/jetbrains/kotlin/fir/backend/Fir2IrClassifierStorage.kt

@@ -72,14 +72,16 @@ class Fir2IrClassifierStorage(
 
     fun preCacheBuiltinClasses() {
         for ((classId, irBuiltinSymbol) in typeConverter.classIdToSymbolMap) {
-            val firClass = classId.toSymbol(session)!!.fir as FirRegularClass
+            // toSymbol() can return null when using an old stdlib that's missing some types
+            val firClass = classId.toSymbol(session)?.fir as FirRegularClass? ?: continue
             val irClass = irBuiltinSymbol.owner
             classCache[firClass] = irClass
             processClassHeader(firClass, irClass)
             declarationStorage.preCacheBuiltinClassMembers(firClass, irClass)
         }
         for ((primitiveClassId, primitiveArrayId) in StandardClassIds.primitiveArrayTypeByElementType) {
-            val firClass = primitiveArrayId.toLookupTag().toSymbol(session)!!.fir as FirRegularClass
+            // toSymbol() can return null when using an old stdlib that's missing some types
+            val firClass = primitiveArrayId.toLookupTag().toSymbol(session)?.fir as FirRegularClass? ?: continue
             val irType = typeConverter.classIdToTypeMap[primitiveClassId]
             val irClass = irBuiltIns.primitiveArrayForType[irType]!!.owner
             classCache[firClass] = irClass

libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/kotlin/org/jetbrains/kotlin/gradle/K2Tests.kt

@@ -63,6 +63,17 @@ class CustomK2Tests : KGPBaseTest() {
         }
     }
 
+    @GradleTest
+    @DisplayName("HMPP compilation with JS target and old stdlib. KT-59151")
+    fun testHmppCompilationWithJsAndOldStdlib(gradleVersion: GradleVersion) {
+        with(project("k2-mpp-js-old-stdlib", gradleVersion, buildOptions = defaultBuildOptions.copy(languageVersion = "2.0"))) {
+            val taskToExecute = ":compileKotlinJs"
+            build(taskToExecute) {
+                assertTasksExecuted(taskToExecute)
+            }
+        }
+    }
+
     @GradleTest
     @DisplayName("Native metadata of intermediate with reference to internal in common. KT-58219")
     fun nativeMetadataOfIntermediateWithReferenceToInternalInCommon(gradleVersion: GradleVersion) {

libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/resources/testProject/k2-mpp-js-old-stdlib/build.gradle.kts

@@ -0,0 +1,32 @@
+import org.jetbrains.kotlin.gradle.dsl.KotlinVersion
+import org.jetbrains.kotlin.gradle.tasks.KotlinCompilationTask
+
+plugins {
+    kotlin("multiplatform")
+}
+
+repositories {
+	mavenLocal()
+    mavenCentral()
+}
+
+kotlin {
+    js {
+        nodejs()
+        browser()
+    }
+
+    sourceSets {
+        val commonMain by getting {
+            dependencies {
+                implementation(kotlin("stdlib-common", "1.6.21"))
+            }
+        }
+
+        val jsMain by getting {
+            dependencies {
+                implementation(kotlin("stdlib-js", "1.6.21"))
+            }
+        }
+    }
+}

libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/resources/testProject/k2-mpp-js-old-stdlib/gradle.properties

@@ -0,0 +1 @@
+kotlin.stdlib.default.dependency=false

libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/resources/testProject/k2-mpp-js-old-stdlib/src/jsMain/kotlin/main.kt

@@ -0,0 +1 @@
+fun main() {}