Rename Closure instance field type_arguments_ to instantiator_.

runtime/vm/stub_code_x64.cc

Index: runtime/vm/stub_code_x64.cc
diff --git a/runtime/vm/stub_code_x64.cc b/runtime/vm/stub_code_x64.cc
index a2f123e3a782b2d626099acf3b264378b31fa80c..f3a7f12ab8c0e314a2a2529ea9988b46322ea88d 100644
--- a/runtime/vm/stub_code_x64.cc
+++ b/runtime/vm/stub_code_x64.cc
@@ -1747,11 +1747,12 @@ static void GenerateSubtypeNTestCacheStub(Assembler* assembler, int n) {
   __ addq(RDX, Immediate(Array::data_offset() - kHeapObjectTag));
   // RDX: Entry start.
   // R10: instance class id.
-  // R13: instance type arguments.
+  // R13: instance type arguments (still null if closure).
   Label loop, found, not_found, next_iteration;
   __ SmiTag(R10);
   __ cmpq(R10, Immediate(Smi::RawValue(kClosureCid)));
   __ j(NOT_EQUAL, &loop, Assembler::kNearJump);
+  __ movq(R13, FieldAddress(RAX, Closure::instantiator_offset()));
   __ movq(R10, FieldAddress(RAX, Closure::function_offset()));
   // R10: instance class id as Smi or function.
   __ Bind(&loop);