JS: minor, do not use '==' on descriptors from built-ins

Use KotlinBuiltIns.isString instead of equality with
KotlinBuiltIns.string, which is more portable across different module
configurations. Also use isSubtypeOf instead of
DescriptorUtils.isSubclass and thus get rid of an extra error on an
unresolved class in nativeAnnotationCheckers.kt

js/js.frontend/src/org/jetbrains/kotlin/js/resolve/nativeAnnotationCheckers.kt

@@ -27,8 +27,8 @@ import org.jetbrains.kotlin.js.translate.utils.AnnotationsUtils
 import org.jetbrains.kotlin.psi.KtDeclaration
 import org.jetbrains.kotlin.psi.KtNamedFunction
 import org.jetbrains.kotlin.resolve.DescriptorUtils
-import org.jetbrains.kotlin.resolve.checkers.DeclarationCheckerContext
 import org.jetbrains.kotlin.resolve.checkers.DeclarationChecker
+import org.jetbrains.kotlin.resolve.checkers.DeclarationCheckerContext
 import org.jetbrains.kotlin.resolve.descriptorUtil.builtIns
 import org.jetbrains.kotlin.types.TypeUtils
 import org.jetbrains.kotlin.types.typeUtil.isSubtypeOf
@@ -68,12 +68,12 @@ internal abstract class AbstractNativeIndexerChecker(
     override fun additionalCheck(declaration: KtNamedFunction, descriptor: FunctionDescriptor, diagnosticHolder: DiagnosticSink) {
         val parameters = descriptor.valueParameters
         val builtIns = descriptor.builtIns
-        if (parameters.size > 0) {
-            val firstParamClassDescriptor = DescriptorUtils.getClassDescriptorForType(parameters.get(0).type)
-            if (firstParamClassDescriptor != builtIns.string &&
-                !DescriptorUtils.isSubclass(firstParamClassDescriptor, builtIns.number)
-            ) {
-                diagnosticHolder.report(ErrorsJs.NATIVE_INDEXER_KEY_SHOULD_BE_STRING_OR_NUMBER.on(declaration.valueParameters.first(), indexerKind))
+        if (parameters.isNotEmpty()) {
+            val firstParamType = parameters.first().type
+            if (!KotlinBuiltIns.isString(firstParamType) && !firstParamType.isSubtypeOf(builtIns.number.defaultType)) {
+                diagnosticHolder.report(
+                    ErrorsJs.NATIVE_INDEXER_KEY_SHOULD_BE_STRING_OR_NUMBER.on(declaration.valueParameters.first(), indexerKind)
+                )
             }
         }