[vm] Clear native flag on dynamic invocation forwarders.

When creating a dynamic invocation forwarder, the VM clones the original
function object and then modifies the clone to create the forwarder.
Previously, the is_native flag was left untouched, but now it is
properly cleared.

Change-Id: I133ffdc0d0d3d727f7b7e47af22944e6b0574ee7
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/110721
Reviewed-by: Martin Kustermann <kustermann@google.com>
Commit-Queue: Teagan Strickland <sstrickl@google.com>

runtime/vm/object.cc

@@ -3074,6 +3074,7 @@ RawFunction* Function::CreateDynamicInvocationForwarder(
   forwarder ^= Object::Clone(*this, Heap::kOld);
 
   forwarder.set_name(mangled_name);
+  forwarder.set_is_native(false);
   forwarder.set_kind(RawFunction::kDynamicInvocationForwarder);
   forwarder.set_is_debuggable(false);