Don't assume at least one closure function is encountered before tree-shaking. Allow…

runtime/vm/object.cc

Index: runtime/vm/object.cc
diff --git a/runtime/vm/object.cc b/runtime/vm/object.cc
index ba7b223da2fd50d0cd096c3cc60050541de50a32..faf0ecac1ce3e33833237837f5f46ce2bf3c5be3 100644
--- a/runtime/vm/object.cc
+++ b/runtime/vm/object.cc
@@ -2338,6 +2338,11 @@ RawFunction* Class::InvocationDispatcherFunctionFromIndex(intptr_t idx) const {
 }
 
 
+void Class::set_closures(const GrowableObjectArray& value) const {
+  StorePointer(&raw_ptr()->closure_functions_, value.raw());
+}
+
+
 void Class::AddClosureFunction(const Function& function) const {
   GrowableObjectArray& closures =
       GrowableObjectArray::Handle(raw_ptr()->closure_functions_);