[url_launcher] launchUrl always returns true for valid schemes on the web. (#7229)

## Description

Since `noopener` is used to open URLs, there is no way to determine if the browser has succesfully opened a given URL ([MDN](https://developer.mozilla.org/en-US/docs/Web/API/Window/open#noopener)). Therefore success is now assumed for supported schemes.

Fixes flutter/flutter#139783.

Implementation largely follows suggestions by @ditman written down in [a comment](flutter/flutter#139783 (comment)).

The only exception is that `launchUrl` does not returns a hard-coded `true`, but a `canLaunch(url)`. That way `false` is returned for invalid schemes (e.g. `javascript:`, as is checked in [an integration test](https://github.com/Frank3K/flutter_packages/blob/0c29f9463acb09a5ad8f1ada9ff16e7b8221bf68/packages/url_launcher/url_launcher_web/example/integration_test/url_launcher_web_test.dart#L119-L122)).

## Testing

### Sample application

The following application can be used to validate the fix. Note that the `print` is executed using the current version of url_launcher_web (2.3.1) and that it does not execute when the fix from this PR is used.

`main.dart`

```dart
import 'package:flutter/material.dart';
import 'package:url_launcher/url_launcher.dart';

final Uri _url = Uri.parse('https://flutter.dev');

void main() => runApp(
      const MaterialApp(
        home: Material(
          child: Center(
            child: ElevatedButton(
              onPressed: _launchUrl,
              child: Text('Show Flutter homepage'),
            ),
          ),
        ),
      ),
    );

Future<void> _launchUrl() async {
  if (!await launchUrl(_url)) {
    // ignore: avoid_print
    print('Failed opening link.');
  }
}
```

Author: Frank3K

packages/url_launcher/url_launcher_web/CHANGELOG.md

@@ -1,3 +1,7 @@
+## 2.3.3
+
+* Changes `launchUrl` so it always returns `true`, except for disallowed URL schemes.
+
 ## 2.3.2
 
 * Adds support for `web: ^1.0.0`.

packages/url_launcher/url_launcher_web/README.md

@@ -30,4 +30,14 @@ In such cases, you can use the `webOnlyWindowName` parameter, setting it to
 `_self`, to open the URL within the current tab. Another approach is to ensure
 that the `uri` is synchronously ready.
 
-Read more: MDN > [Transient activation](https://developer.mozilla.org/en-US/docs/Glossary/Transient_activation).
+Read more: MDN > [Transient activation](https://developer.mozilla.org/en-US/docs/Glossary/Transient_activation).
+
+### Method `launchUrl` always returns `true` for allowed schemes
+
+The `launchUrl` method always returns `true` on the web platform for allowed
+schemes. This is because URLs are opened in a new window using the `noopener`
+window feature. When the `noopener` feature is used, the browser does not 
+return any information that can be used to determine if the link was 
+successfully opened.
+
+Read more: MDN > [window.open](https://developer.mozilla.org/en-US/docs/Web/API/Window/open#noopener).

packages/url_launcher/url_launcher_web/example/integration_test/url_launcher_web_test.dart

@@ -120,6 +120,15 @@ void main() {
           (WidgetTester _) async {
         expect(plugin.launch('javascript:alert("1")'), completion(isFalse));
       });
+
+      testWidgets('launching a unknown sceheme returns true',
+          (WidgetTester _) async {
+        expect(
+            plugin.launch(
+              'foo:bar',
+            ),
+            completion(isTrue));
+      });
     });
 
     group('openNewWindow', () {

packages/url_launcher/url_launcher_web/lib/url_launcher_web.dart

@@ -66,25 +66,30 @@ class UrlLauncherPlugin extends UrlLauncherPlatform {
 
   /// Opens the given [url] in the specified [webOnlyWindowName].
   ///
-  /// Returns the newly created window.
+  /// Always returns `true`, except for disallowed schemes. Because `noopener`
+  /// is used as a window feature, it can not be detected if the window was
+  /// opened successfully.
+  /// See https://html.spec.whatwg.org/multipage/nav-history-apis.html#window-open-steps.
   @visibleForTesting
-  html.Window? openNewWindow(String url, {String? webOnlyWindowName}) {
+  bool openNewWindow(String url, {String? webOnlyWindowName}) {
     final String? scheme = _getUrlScheme(url);
     // Actively disallow opening some schemes, like javascript.
     // See https://github.com/flutter/flutter/issues/136657
     if (_isDisallowedScheme(scheme)) {
       if (kDebugMode) {
         print('Disallowed URL with scheme: $scheme');
       }
-      return null;
+      return false;
     }
     // Some schemes need to be opened on the _top window context on Safari.
     // See https://github.com/flutter/flutter/issues/51461
     final String target = webOnlyWindowName ??
         ((_isSafari && _isSafariTargetTopScheme(scheme)) ? '_top' : '');
 
     // ignore: unsafe_html
-    return _window.open(url, target, 'noopener,noreferrer');
+    _window.open(url, target, 'noopener,noreferrer');
+
+    return true;
   }
 
   @override
@@ -109,7 +114,7 @@ class UrlLauncherPlugin extends UrlLauncherPlatform {
   @override
   Future<bool> launchUrl(String url, LaunchOptions options) async {
     final String? windowName = options.webOnlyWindowName;
-    return openNewWindow(url, webOnlyWindowName: windowName) != null;
+    return openNewWindow(url, webOnlyWindowName: windowName);
   }
 
   @override

packages/url_launcher/url_launcher_web/pubspec.yaml

@@ -2,7 +2,7 @@ name: url_launcher_web
 description: Web platform implementation of url_launcher
 repository: https://github.com/flutter/packages/tree/main/packages/url_launcher/url_launcher_web
 issue_tracker: https://github.com/flutter/flutter/issues?q=is%3Aissue+is%3Aopen+label%3A%22p%3A+url_launcher%22
-version: 2.3.2
+version: 2.3.3
 
 environment:
   sdk: ^3.3.0