Work around issue 23008 in pub.

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 4fe118eb25c238e8df4dc24be2c8b0757e6f8982..12ee4e264623b0f98302a2c9f1a4aaec1ae99682 100644
--- a/sdk/lib/_internal/pub/lib/src/http.dart
+++ b/sdk/lib/_internal/pub/lib/src/http.dart
@@ -107,6 +107,9 @@ class _PubHttpClient extends http.BaseClient {
         throw new PubHttpException(response);
       });
     }).catchError((error, stackTrace) {
+      // Work around issue 23008.
+      if (stackTrace == null) stackTrace = new Chain.current();
+
       if (error is SocketException &&
           error.osError != null) {
         if (error.osError.errorCode == 8 ||