Static warning fix.

pkgs/package_config/lib/packages_file.dart

diff --git a/pkgs/package_config/lib/packages_file.dart b/pkgs/package_config/lib/packages_file.dart
index 25d2d68..3f432aa 100644
--- a/pkgs/package_config/lib/packages_file.dart
+++ b/pkgs/package_config/lib/packages_file.dart
@@ -109,7 +109,7 @@
     }
     if (uri.scheme == "package") {
       throw new ArgumentError.value(
-          "Package location must not be a package: URI", uri);
+          "Package location must not be a package: URI", uri.toString());
     }
     output.write(packageName);
     output.write(':');