Assert that url is non-null in NetworkImage (#9626)

packages/flutter/lib/src/services/image_provider.dart

@@ -311,7 +311,7 @@ class NetworkImage extends ImageProvider<NetworkImage> {
   /// Creates an object that fetches the image at the given URL.
   ///
   /// The arguments must not be null.
-  const NetworkImage(this.url, { this.scale: 1.0 });
+  const NetworkImage(this.url, { this.scale: 1.0 }) : assert(url != null);
 
   /// The URL from which the image will be fetched.
   final String url;

packages/flutter/test/services/image_provider_test.dart

+// Copyright 2017 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter/services.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  test('NetworkImage non-null url test', () {
+    expect(() {
+      new NetworkImage(null); // ignore: prefer_const_constructors
+    }, throwsAssertionError);
+  });
+}