warnings and hints not updated after commenting out a class in an imported file

[skybrian]

To reproduce, create two files:

import "target.dart";

main() {
  print(new Target());
}

target.dart:

class Target {}

Open both files in Atom. Comment out the class definition for Target.

The first issue is that no error shows up before saving target.dart.

Next, save the file. This warning appears:
Undefined class 'Target - 'main.dart, line 4

This is correct, however there should also be a hint that target.dart is now an unused import.

Reanalyze sources, and the hint now appears:
Unused import - main.dart, line 1

(Found via a stress test.)