[dart2js] New RTI: Don't try to instantiate a null function.

Change-Id: I6532fb138343e8fe01b3773b3ef6693e52b1b312
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/118364
Commit-Queue: Mayank Patke <fishythefish@google.com>
Reviewed-by: Stephen Adams <sra@google.com>

sdk/lib/_internal/js_runtime/lib/rti.dart

@@ -363,6 +363,11 @@ Rti evalInInstance(instance, String recipe) {
 @pragma('dart2js:noInline')
 Rti instantiatedGenericFunctionType(
     Rti genericFunctionRti, Rti instantiationRti) {
+  // If --lax-runtime-type-to-string is enabled and we never check the function
+  // type, then the function won't have a signature, so its RTI will be null. In
+  // this case, there is nothing to instantiate, so we return `null` and the
+  // instantiation appears to be an interface type instead.
+  if (genericFunctionRti == null) return null;
   var bounds = Rti._getGenericFunctionBounds(genericFunctionRti);
   var typeArguments = Rti._getInterfaceTypeArguments(instantiationRti);
   assert(_Utils.arrayLength(bounds) == _Utils.arrayLength(typeArguments));

sdk_nnbd/lib/_internal/js_runtime/lib/rti.dart

@@ -365,6 +365,11 @@ Rti evalInInstance(instance, String recipe) {
 @pragma('dart2js:noInline')
 Rti instantiatedGenericFunctionType(
     Rti genericFunctionRti, Rti instantiationRti) {
+  // If --lax-runtime-type-to-string is enabled and we never check the function
+  // type, then the function won't have a signature, so its RTI will be null. In
+  // this case, there is nothing to instantiate, so we return `null` and the
+  // instantiation appears to be an interface type instead.
+  if (genericFunctionRti == null) return null;
   var bounds = Rti._getGenericFunctionBounds(genericFunctionRti);
   var typeArguments = Rti._getInterfaceTypeArguments(instantiationRti);
   assert(_Utils.arrayLength(bounds) == _Utils.arrayLength(typeArguments));