[JS stdlib] Add a safeguard for BigInt class for pre-ES2020 engines

Without this, this code would throw the `ReferenceError`
even when compiling Longs as BigInts is disabled.

^KT-80873 Fixed

Author: broadwaylamb

libraries/stdlib/js/src/kotlin/reflect/primitives.kt

@@ -6,7 +6,6 @@
 package kotlin.reflect.js.internal
 
 import kotlin.internal.UsedFromCompilerGeneratedCode
-import kotlin.js.JsClass
 import kotlin.js.internal.BigInt
 
 @JsName("PrimitiveClasses")
@@ -34,7 +33,8 @@ internal object PrimitiveClasses {
     val intClass = PrimitiveKClassImpl(js("Number").unsafeCast<JsClass<Int>>(), "Int", { it is Int })
 
     @JsName("longClass")
-    val longClass = PrimitiveKClassImpl(BigInt.unsafeCast<JsClass<Long>>(), "Long", { it is Long })
+    val longClass =
+        PrimitiveKClassImpl((if (jsTypeOf(BigInt) == "undefined") VOID else BigInt).unsafeCast<JsClass<Long>>(), "Long", { it is Long })
 
     @JsName("floatClass")
     val floatClass = PrimitiveKClassImpl(js("Number").unsafeCast<JsClass<Float>>(), "Float", { it is Float })