Add a missing import to pub.

R=rnystrom@google.com

Review URL: https://codereview.chromium.org//1047013003

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@44790 260f80e4-7a28-3924-810f-c04153c831b5

lib/src/http.dart

@@ -11,6 +11,7 @@ import 'dart:io';
 
 import 'package:http/http.dart' as http;
 import 'package:http_throttle/http_throttle.dart';
+import 'package:stack_trace/stack_trace.dart';
 
 import 'io.dart';
 import 'log.dart' as log;