Remove signature classes from the VM.

runtime/vm/intrinsifier_mips.cc

Index: runtime/vm/intrinsifier_mips.cc
diff --git a/runtime/vm/intrinsifier_mips.cc b/runtime/vm/intrinsifier_mips.cc
index 2be111456444766dbeb15eed50197184ae94f51e..1e289cc6cf33e873c9529afe68d6e4aaf52907c4 100644
--- a/runtime/vm/intrinsifier_mips.cc
+++ b/runtime/vm/intrinsifier_mips.cc
@@ -1646,12 +1646,10 @@ void Intrinsifier::ObjectRuntimeType(Assembler* assembler) {
   Label fall_through;
   __ lw(T0, Address(SP, 0 * kWordSize));
   __ LoadClassIdMayBeSmi(T1, T0);
+  __ BranchEqual(T1, Immediate(kClosureCid), &fall_through);
   __ LoadClassById(T2, T1);
   // T2: class of instance (T0).
 
-  __ lw(T1, FieldAddress(T2, Class::signature_function_offset()));
-  __ BranchNotEqual(T1, Object::null_object(), &fall_through);
-
   __ lhu(T1, FieldAddress(T2, Class::num_type_arguments_offset()));
   __ BranchNotEqual(T1, Immediate(0), &fall_through);