Include duplicate package name in error message (dart-lang/package_config#104)

pkgs/package_config/lib/src/package_config_impl.dart

diff --git a/pkgs/package_config/lib/src/package_config_impl.dart b/pkgs/package_config/lib/src/package_config_impl.dart
index c751115..1229991 100644
--- a/pkgs/package_config/lib/src/package_config_impl.dart
+++ b/pkgs/package_config/lib/src/package_config_impl.dart
@@ -77,7 +77,7 @@
       var name = package.name;
       if (packageNames.contains(name)) {
         onError(PackageConfigArgumentError(
-            name, "packages", "Duplicate package name"));
+            name, "packages", "Duplicate package name '$name'"));
         continue;
       }
       packageNames.add(name);