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

runtime/vm/object.h

Index: runtime/vm/object.h
diff --git a/runtime/vm/object.h b/runtime/vm/object.h
index db42cf84675cd0efe042e0a80496f26aad57ae21..df1953f20d253e41ee7eab54e93bde884cce0c22 100644
--- a/runtime/vm/object.h
+++ b/runtime/vm/object.h
@@ -1183,6 +1183,7 @@ class Class : public Object {
   RawGrowableObjectArray* closures() const {
     return raw_ptr()->closure_functions_;
   }
+  void set_closures(const GrowableObjectArray& value) const;
   void AddClosureFunction(const Function& function) const;
   RawFunction* LookupClosureFunction(intptr_t token_pos) const;
   intptr_t FindClosureIndex(const Function& function) const;