Use url_launcher plugin in Flutter Gallery (#9751)

examples/flutter_gallery/android/app/src/main/java/io/flutter/examples/gallery/MainActivity.java

 package io.flutter.examples.gallery;
 
 import android.os.Bundle;
+
 import io.flutter.app.FlutterActivity;
 import io.flutter.plugins.PluginRegistry;
 

examples/flutter_gallery/lib/gallery/drawer.dart

@@ -7,14 +7,15 @@ import 'dart:math' as math;
 import 'package:flutter/foundation.dart' show defaultTargetPlatform, required;
 import 'package:flutter/gestures.dart';
 import 'package:flutter/material.dart';
-import 'package:flutter/services.dart';
+
+import 'package:url_launcher/url_launcher.dart';
 
 class LinkTextSpan extends TextSpan {
   LinkTextSpan({ TextStyle style, String url, String text }) : super(
     style: style,
     text: text ?? url,
     recognizer: new TapGestureRecognizer()..onTap = () {
-      UrlLauncher.launch(url);
+      launch(url);
     }
   );
 }
@@ -203,7 +204,7 @@ class GalleryDrawer extends StatelessWidget {
       leading: const Icon(Icons.report),
       title: const Text('Send feedback'),
       onTap: onSendFeedback ?? () {
-        UrlLauncher.launch('https://github.com/flutter/flutter/issues/new');
+        launch('https://github.com/flutter/flutter/issues/new');
       },
     );
 

examples/flutter_gallery/pubspec.yaml

@@ -6,6 +6,7 @@ dependencies:
 
   flutter:
     sdk: flutter
+  url_launcher: ^0.3.1
 
   # Also update dev/benchmarks/complex_layout/pubspec.yaml
   flutter_gallery_assets: