It turns out Macromedia just used an undocumented but user-modifiable property called __constructor__ to store the constructor.

A previous version of this PR (whose history has been scrubbed, but go check 918d88a if you're curious) implemented a new `TObject` property which basically every line of code that dealt with object construction had to populate. It was terrible.

Author: kmeisthax

core/src/avm1.rs

@@ -1590,6 +1590,7 @@ impl<'gc> Avm1<'gc> {
             ScriptObject::object(context.gc_context, Some(super_proto)).into();
 
         sub_prototype.set("constructor", superclass.into(), self, context)?;
+        sub_prototype.set("__constructor__", superclass.into(), self, context)?;
         subclass.set("prototype", sub_prototype.into(), self, context)?;
 
         Ok(())
@@ -2077,6 +2078,7 @@ impl<'gc> Avm1<'gc> {
 
         let this = prototype.new(self, context, prototype, &args)?;
 
+        this.set("__constructor__", constructor.into(), self, context)?;
         if self.current_swf_version() < 7 {
             this.set("constructor", constructor.into(), self, context)?;
         }
@@ -2119,6 +2121,7 @@ impl<'gc> Avm1<'gc> {
                 {
                     let this = prototype.new(self, context, prototype, &args)?;
 
+                    this.set("__constructor__", constructor.into(), self, context)?;
                     if self.current_swf_version() < 7 {
                         this.set("constructor", constructor.into(), self, context)?;
                     }

core/src/avm1/super_object.rs

@@ -68,7 +68,7 @@ impl<'gc> SuperObject<'gc> {
     ) -> Result<Option<Object<'gc>>, Error> {
         if let Some(super_proto) = self.super_proto() {
             Ok(super_proto
-                .get("constructor", avm, context)?
+                .get("__constructor__", avm, context)?
                 .resolve(avm, context)?
                 .as_object()
                 .ok())
@@ -181,7 +181,9 @@ impl<'gc> TObject<'gc> for SuperObject<'gc> {
     }
 
     fn set_proto(&self, gc_context: MutationContext<'gc, '_>, prototype: Option<Object<'gc>>) {
-        self.0.write(gc_context).proto = prototype;
+        if let Some(prototype) = prototype {
+            self.0.write(gc_context).base_proto = prototype;
+        }
     }
 
     fn define_value(