Make pub generate .packages file.

lib/src/entrypoint.dart

Index: lib/src/entrypoint.dart
diff --git a/lib/src/entrypoint.dart b/lib/src/entrypoint.dart
index e97f688e5b66d96da849a773965c4ecbc53be991..2dc3d136e0834f092887201a4fc902584169f91e 100644
--- a/lib/src/entrypoint.dart
+++ b/lib/src/entrypoint.dart
@@ -15,6 +15,7 @@ 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';
@@ -151,6 +152,8 @@ class Entrypoint {
       // dependencies, it shouldn't fail unless that fails.
       log.exception(error, stackTrace);
     }
+
+    writePackagesMap(_packageGraph);
   }
 
   /// Precompile any transformed dependencies of the entrypoint.