[vm] Relax recently added assertion in Dart 1 mode

This CL relaxes assertion added recently in
c6c4748 as it could fail in Dart 1
mode.

This should fix 'checked vm tests' step on debug bots which still runs
tests in Dart 1 mode.

Change-Id: Ifdd6e27087b44822ce9cd5be48750af1c7d45a8e
Reviewed-on: https://dart-review.googlesource.com/75201
Reviewed-by: Zach Anderson <zra@google.com>
Reviewed-by: Régis Crelier <regis@google.com>
Commit-Queue: Alexander Markov <alexmarkov@google.com>

Author: alexmarkov

runtime/vm/compiler/backend/il.h

@@ -2993,7 +2993,7 @@ class AssertAssignableInstr : public TemplateDefinition<3, Throws, Pure> {
     ASSERT(!dst_type.IsNull());
     ASSERT(!dst_type.IsTypeRef());
     ASSERT(!dst_name.IsNull());
-    ASSERT(!dst_type.IsDynamicType());
+    ASSERT(!FLAG_strong || !dst_type.IsDynamicType());
     SetInputAt(0, value);
     SetInputAt(1, instantiator_type_arguments);
     SetInputAt(2, function_type_arguments);