Generate the package spec from the lockfile.

lib/src/entrypoint.dart

Index: lib/src/entrypoint.dart
diff --git a/lib/src/entrypoint.dart b/lib/src/entrypoint.dart
index 7e90814c8780045c95f739ba5a64464e286683fd..10c76177c26f5220e1508c10cf69d6f570aa4c15 100644
--- a/lib/src/entrypoint.dart
+++ b/lib/src/entrypoint.dart
@@ -15,7 +15,6 @@ import 'lock_file.dart';
 import 'log.dart' as log;
 import 'package.dart';
 import 'package_graph.dart';
-import 'package_locations.dart';
 import 'sdk.dart' as sdk;
 import 'solver/version_solver.dart';
 import 'source/cached.dart';
@@ -156,7 +155,7 @@ class Entrypoint {
       log.exception(error, stackTrace);
     }
 
-    writePackagesMap(_packageGraph);
+    writeTextFile(packagesFile, lockFile.packagesFile(root.name));
   }
 
   /// Precompile any transformed dependencies of the entrypoint.