Run "pub run" against the .packages file.

lib/src/entrypoint.dart

Index: lib/src/entrypoint.dart
diff --git a/lib/src/entrypoint.dart b/lib/src/entrypoint.dart
index ceb277903edffe9ffe847de824d034602feea06f..8e8b7921fd15105722405e9d94c5d21f2b332af3 100644
--- a/lib/src/entrypoint.dart
+++ b/lib/src/entrypoint.dart
@@ -76,6 +76,9 @@ class Entrypoint {
   /// The path to the entrypoint's "packages" directory.
   String get packagesDir => root.path('packages');
 
+  /// The path to the entrypoint's ".packages" file.
+  String get packagesFile => root.path('.packages');
+
   /// `true` if the entrypoint package currently has a lock file.
   bool get lockFileExists => _lockFile != null || entryExists(lockFilePath);