Use class id instead of class in subtype test cache.

runtime/vm/object_test.cc

Index: runtime/vm/object_test.cc
===================================================================
--- runtime/vm/object_test.cc	(revision 9519)
+++ runtime/vm/object_test.cc	(working copy)
@@ -2640,14 +2640,14 @@
   EXPECT_EQ(0, cache.NumberOfChecks());
   const TypeArguments& targ_0 = TypeArguments::Handle(TypeArguments::New(2));
   const TypeArguments& targ_1 = TypeArguments::Handle(TypeArguments::New(3));
-  cache.AddCheck(empty_class, targ_0, targ_1, Bool::Handle(Bool::True()));
+  cache.AddCheck(empty_class.id(), targ_0, targ_1, Bool::Handle(Bool::True()));
   EXPECT_EQ(1, cache.NumberOfChecks());
-  Class& test_class = Class::Handle();
+  intptr_t test_class_id = -1;
   AbstractTypeArguments& test_targ_0 = AbstractTypeArguments::Handle();
   AbstractTypeArguments& test_targ_1 = AbstractTypeArguments::Handle();
   Bool& test_result = Bool::Handle();
-  cache.GetCheck(0, &test_class, &test_targ_0, &test_targ_1, &test_result);
-  EXPECT_EQ(empty_class.raw(), test_class.raw());
+  cache.GetCheck(0, &test_class_id, &test_targ_0, &test_targ_1, &test_result);
+  EXPECT_EQ(empty_class.id(), test_class_id);
   EXPECT_EQ(targ_0.raw(), test_targ_0.raw());
   EXPECT_EQ(targ_1.raw(), test_targ_1.raw());
   EXPECT_EQ(Bool::True(), test_result.raw());