Adding a CollisionShape2D on an instanced Area2D in the editor does not work

[chanon]

Operating system or device:

Windows 10

Issue description (what happened, and what was expected):

In TestArea.tscn I have

TestArea (Area2D)

With script TestArea.gd:

extends Area2D

func _input_event(viewport, event, shape_idx):
    print(self.get_name())

If I add that to another scene main.tscn and add a CollisionShape2D to it:

Node2D
     -> TestArea (from TestArea.tscn)
               -> CollisionShape2D (added in main.tscn with rectangle shape) 

The CollisionShape2D doesn't seem to work and nothing is logged out.

I have attached an example project. To the left is an Area2D that was created in main.tscn. On the right is a TestArea loaded from TestArea.tscn
collision_shape.zip

[leonkrause]

The issue is the same as #2314, so let's use that one