[web] Ensure Flutter adds a generator meta-tag. (#55714)

Adds a `meta name="generator" content="Flutter"` tag when the engine UI initializes.

## Issues

Fixes flutter/flutter#156262

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style

Author: ditman

lib/web_ui/lib/src/engine/initialization.dart

@@ -229,6 +229,9 @@ Future<void> initializeEngineUi() async {
   RawKeyboard.initialize(onMacOs: ui_web.browser.operatingSystem == ui_web.OperatingSystem.macOs);
   KeyboardBinding.initInstance();
 
+  // Ensures Flutter renders a global "generator" meta-tag.
+  ensureMetaTag('generator', 'Flutter');
+
   if (!configuration.multiViewEnabled) {
     final EngineFlutterWindow implicitView =
         ensureImplicitViewInitialized(hostElement: configuration.hostElement);

lib/web_ui/lib/src/engine/util.dart

@@ -722,6 +722,19 @@ void setThemeColor(ui.Color? color) {
   }
 }
 
+/// Ensure a "meta" tag with [name] and [content] is set on the page.
+void ensureMetaTag(String name, String content) {
+  final DomElement? existingTag =
+    domDocument.querySelector('meta[name=$name][content=$content]');
+
+  if (existingTag == null) {
+    final DomHTMLMetaElement meta = createDomHTMLMetaElement()
+        ..name = name
+        ..content = content;
+    domDocument.head!.append(meta);
+  }
+}
+
 bool? _ellipseFeatureDetected;
 
 /// Draws CanvasElement ellipse with fallback.

lib/web_ui/test/engine/initialization_test.dart

@@ -8,6 +8,7 @@ import 'package:js/js_util.dart' as js_util;
 import 'package:test/bootstrap/browser.dart';
 import 'package:test/test.dart';
 import 'package:ui/src/engine.dart' as engine;
+import 'package:ui/src/engine/dom.dart';
 import 'package:ui/ui_web/src/ui_web.dart' as ui_web;
 
 @JS('_flutter')
@@ -76,6 +77,10 @@ void testMain() {
     // Check that the object we captured is actually a loader
     expect(pluginsRegistered, isTrue, reason: 'Plugins should be immediately registered in autoStart mode.');
     expect(appRan, isTrue, reason: 'App should run immediately in autoStart mode');
+
+    // After starting the engine, the meta-generator tag should be on the page
+    final DomElement? meta = domDocument.querySelector('meta[name=generator][content=Flutter]');
+    expect(meta, isNotNull, reason: 'The generator meta-tag should be added when Flutter initializes its UI.');
   });
 
   // We cannot test anymore, because by now the engine has registered some stuff that can't be rewound back.