Make pub generate .packages file.

test/get/dry_run_does_not_apply_changes_test.dart

Index: test/get/dry_run_does_not_apply_changes_test.dart
diff --git a/test/get/dry_run_does_not_apply_changes_test.dart b/test/get/dry_run_does_not_apply_changes_test.dart
index 7a07d25a6d71a1b9767a76b07f7f02855ef9a5cb..8c48cd049f9ce43ab68e0a5d57b31ba2e07fa999 100644
--- a/test/get/dry_run_does_not_apply_changes_test.dart
+++ b/test/get/dry_run_does_not_apply_changes_test.dart
@@ -27,7 +27,9 @@ main() {
       // The lockfile should not be created.
       d.nothing("pubspec.lock"),
       // The "packages" directory should not have been generated.
-      d.nothing("packages")
+      d.nothing("packages"),
+      // The ".packages" file should not have been created.
+      d.nothing(".packages"),
     ]).validate();
   });
 }