Reapply r18377 it was reverted due to the unrelated bug it surfaced.

runtime/vm/object.cc

Index: runtime/vm/object.cc
diff --git a/runtime/vm/object.cc b/runtime/vm/object.cc
index 5ed01da4238e3e232df74d24283e39fa627986a4..c7c2970dde313d8a11a196207ee78e037fa05dda 100644
--- a/runtime/vm/object.cc
+++ b/runtime/vm/object.cc
@@ -2185,12 +2185,13 @@ bool Class::TypeTest(
     // Since we do not truncate the type argument vector of a subclass (see
     // below), we only check a prefix of the proper length.
     // Check for covariance.
-    if (other_type_arguments.IsNull() ||
-        other_type_arguments.IsRawInstantiatedRaw(len)) {
+    if (other_type_arguments.IsNull() || other_type_arguments.IsRaw(len)) {
       return true;
     }
-    if (type_arguments.IsNull() ||
-        type_arguments.IsRawInstantiatedRaw(len)) {
+    if (type_arguments.IsNull() || type_arguments.IsRaw(len)) {
+      // Other type can't be more specific than this one because for that
+      // it would have to have all dynamic type arguments which is checked
+      // above.
       return test_kind == kIsSubtypeOf;
     }
     return type_arguments.TypeTest(test_kind,