ioClient : set a default Client

dart/lib/src/io_client.dart

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'package:http/http.dart';
 import 'package:sentry/sentry.dart';
 
 /// A pure Dart client for Sentry.io crash reporting.
@@ -16,6 +17,7 @@ class SentryIOClient extends SentryClient {
   /// Instantiates a client using [SentryOptions]
   factory SentryIOClient(SentryOptions options) {
     options.sdk ??= Sdk(name: sdkName, version: sdkVersion);
+    options.httpClient ??= Client();
     return SentryIOClient._(options);
   }