[bugfix] Fix proto return value for native classes

Native classes end up calling the nearest class constructor, and with
the recent refactor those class constructors return their own prototype
instead of the class instance's. This breaks certain functions that
currently rely on the `proto` function return the prototype of the
instance itself. We're seeing failures in `@ember-decorators/data`
due to this bug.

This reverts to returning the protoype on the instance directly.

packages/ember-runtime/lib/system/core_object.js

@@ -260,7 +260,10 @@ function makeCtor(base) {
       Class.PrototypeMixin.applyPartial(Class.prototype);
     }
 
-    return Class.prototype;
+    // Native classes will call the nearest superclass's proto function,
+    // and proto is expected to return the current instance's prototype,
+    // so we need to return it from `this` instead
+    return this.prototype;
   };
 
   return Class;