Improve the error message for a transformer pubspec error.

sdk/lib/_internal/pub/lib/src/pubspec.dart

Index: sdk/lib/_internal/pub/lib/src/pubspec.dart
diff --git a/sdk/lib/_internal/pub/lib/src/pubspec.dart b/sdk/lib/_internal/pub/lib/src/pubspec.dart
index e6344a4db01736472adc8f0c96034a9f3b97b14a..759f01e19f9f26af5957d2386a5bb2ec52d544ba 100644
--- a/sdk/lib/_internal/pub/lib/src/pubspec.dart
+++ b/sdk/lib/_internal/pub/lib/src/pubspec.dart
@@ -149,7 +149,7 @@ class Pubspec {
         } else {
           if (transformer.length != 1) {
             _error('"$field" must have a single key: the transformer '
-                'identifier.');
+                'identifier. Was "$transformer".');
           } else if (transformer.keys.single is! String) {
             _error('"$field" transformer identifier must be a string, but was '
                 '"$library".');