Work around issue 23008 in pub.

R=rnystrom@google.com

Review URL: https://codereview.chromium.org//1007733004

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@44785 260f80e4-7a28-3924-810f-c04153c831b5

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 ||