Remove signature classes from the VM.

runtime/vm/intrinsifier_x64.cc

Index: runtime/vm/intrinsifier_x64.cc
diff --git a/runtime/vm/intrinsifier_x64.cc b/runtime/vm/intrinsifier_x64.cc
index e3f2ca4d59b8c8608cd31aca435d573fb39a1de2..d9eb08baa9990fa73290a6f4f34b8d817715c57e 100644
--- a/runtime/vm/intrinsifier_x64.cc
+++ b/runtime/vm/intrinsifier_x64.cc
@@ -1536,11 +1536,11 @@ void Intrinsifier::ObjectRuntimeType(Assembler* assembler) {
   __ LoadClassIdMayBeSmi(RCX, RAX);
 
   // RCX: untagged cid of instance (RAX).
+  __ cmpq(RCX, Immediate(kClosureCid));
+  __ j(EQUAL, &fall_through, Assembler::kNearJump);  // Instance is a closure.
+
   __ LoadClassById(RDI, RCX);
   // RDI: class of instance (RAX).
-  __ movq(RCX, FieldAddress(RDI, Class::signature_function_offset()));
-  __ CompareObject(RCX, Object::null_object());
-  __ j(NOT_EQUAL, &fall_through, Assembler::kNearJump);
 
   __ movzxw(RCX, FieldAddress(RDI, Class::num_type_arguments_offset()));
   __ cmpq(RCX, Immediate(0));