Add debugging info to a failing case in the pub tests.

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

Index: sdk/lib/_internal/pub/lib/src/http.dart
diff --git a/sdk/lib/_internal/pub/lib/src/http.dart b/sdk/lib/_internal/pub/lib/src/http.dart
index 9fc338eb031e87c337d6da87579891244e7fb257..f9e63bbb8590eb0eb5f95e8130a1932d1b80514b 100644
--- a/sdk/lib/_internal/pub/lib/src/http.dart
+++ b/sdk/lib/_internal/pub/lib/src/http.dart
@@ -99,6 +99,8 @@ class PubHttpClient extends http.BaseClient {
               '"${request.url.origin}".');
         }
       }
+      print('Error in PubHttpClient.send (issue 12581) error: $error');
+      print('    stacktrace: $stackTrace');
       throw error;
     }), HTTP_TIMEOUT, 'fetching URL "${request.url}"');
   }