Tweak runtimeTypeToString for better return type inference

sdk/lib/_internal/js_runtime/lib/js_rti.dart

Index: sdk/lib/_internal/js_runtime/lib/js_rti.dart
diff --git a/sdk/lib/_internal/js_runtime/lib/js_rti.dart b/sdk/lib/_internal/js_runtime/lib/js_rti.dart
index 2ba517c334044a9ad4cbd6254e05a4821e0ebcf8..de7db233558002a16e2375aa284c7530a769bb20 100644
--- a/sdk/lib/_internal/js_runtime/lib/js_rti.dart
+++ b/sdk/lib/_internal/js_runtime/lib/js_rti.dart
@@ -169,11 +169,7 @@ String runtimeTypeToString(var rti, {String onTypeVariable(int i)}) {
     return rawRtiToJsConstructorName(rti);
   }
   if (rti is int) {
-    if (onTypeVariable == null) {
-      return rti.toString();
-    } else {
-      return onTypeVariable(rti);
-    }
+    return '${onTypeVariable == null ? rti : onTypeVariable(rti)}';
   }
   if (JS('bool', 'typeof #.func != "undefined"', rti)) {
     // If the RTI has typedef equivalence info (via mirrors), use that since the