Report unused files to watch.

pkg/front_end/test/src/incremental/file_state_test.dart

Index: pkg/front_end/test/src/incremental/file_state_test.dart
diff --git a/pkg/front_end/test/src/incremental/file_state_test.dart b/pkg/front_end/test/src/incremental/file_state_test.dart
index aa927f32f030360fb31b8f5478f7fc9a2934e4c5..9e9f391cdbb9f3246d7f021711017fc15b8ad3d5 100644
--- a/pkg/front_end/test/src/incremental/file_state_test.dart
+++ b/pkg/front_end/test/src/incremental/file_state_test.dart
@@ -163,7 +163,7 @@ import "d.dart";
     expect(fsState.fileUris, contains(d));
 
     // No changes after GC.
-    fsState.gc(e);
+    expect(fsState.gc(e), isEmpty);
     expect(fsState.fileUris, contains(e));
     expect(fsState.fileUris, contains(a));
     expect(fsState.fileUris, contains(b));
@@ -178,7 +178,10 @@ import "d.dart";
 import "d.dart";
 ''');
     await eFile.refresh();
-    fsState.gc(e);
+    {
+      var gcFiles = fsState.gc(e);
+      expect(gcFiles.map((file) => file.uri), unorderedEquals([a, c]));
+    }
     expect(fsState.fileUris, contains(e));
     expect(fsState.fileUris, isNot(contains(a)));
     expect(fsState.fileUris, contains(b));