Fixes assert assignable check for DBC

runtime/vm/simulator_dbc.cc

Index: runtime/vm/simulator_dbc.cc
diff --git a/runtime/vm/simulator_dbc.cc b/runtime/vm/simulator_dbc.cc
index 7329114a00f6fe899e2b76a7f240d0ef8362ae50..364df61f4e0a9ca0615be2d3a4bcc4c70444288d 100644
--- a/runtime/vm/simulator_dbc.cc
+++ b/runtime/vm/simulator_dbc.cc
@@ -1634,6 +1634,18 @@ RawObject* Simulator::Call(const Code& code,
     BYTECODE(AssertAssignable, A_D);  // Stack: instance, type args, type, name
     RawObject** args = SP - 3;
     if (args[0] != null_value) {
+      const AbstractType& dst_type =
+          AbstractType::Handle(static_cast<RawAbstractType*>(args[2]));
+      if (dst_type.IsMalformedOrMalbounded()) {
+        SP[1] = args[0];  // instance.
+        SP[2] = args[3];  // name.
+        SP[3] = args[2];  // type.
+        Exit(thread, FP, SP + 4, pc);
+        NativeArguments args(thread, 3, SP + 1, SP - 3);
+        INVOKE_RUNTIME(DRT_BadTypeError, args);
+        UNREACHABLE();
+      }
+
       RawSubtypeTestCache* cache =
           static_cast<RawSubtypeTestCache*>(LOAD_CONSTANT(rD));
       if (cache != null_value) {