Rename Closure instance field type_arguments_ to instantiator_.

runtime/vm/raw_object.h

Index: runtime/vm/raw_object.h
diff --git a/runtime/vm/raw_object.h b/runtime/vm/raw_object.h
index 54a2df28433777455c2c40dd1e4612b2a0171969..85cebf5ffae146ed54d557c3b89cf17243c84f17 100644
--- a/runtime/vm/raw_object.h
+++ b/runtime/vm/raw_object.h
@@ -1782,10 +1782,14 @@ class RawClosure : public RawInstance {
   RAW_HEAP_OBJECT_IMPLEMENTATION(Closure);
 
   RawObject** from() {
-    return reinterpret_cast<RawObject**>(&ptr()->type_arguments_);
+    return reinterpret_cast<RawObject**>(&ptr()->instantiator_);
   }
 
-  RawTypeArguments* type_arguments_;
+  // No instance fields should be declared before the following 3 fields whose
+  // offsets must be identical in Dart and C++.
+
+  // These 3 fields are also declared in the Dart source of class _Closure.
+  RawTypeArguments* instantiator_;
   RawFunction* function_;
   RawContext* context_;