check-syntax only draws an arrow when an additional definition is present in unit

[dfeltey]

#lang racket

(unit
  (import)
  (export)
  (define x 1)
  (define y 2)
  x)

Draws the arrows I expect, but in

#lang racket

(unit
  (import)
  (export)
  (define x 1)
  x)

check-syntax reports no bound occurences of x. Even though the expansion has a disappeared-binding property for x.

[dfeltey]

This program also draws the correct arrows, somewhat surprisingly based on the above examples,

(unit
  (import)
  (export)
  (define x 1)
   (+ x x))