Add didSendFirstFrameEvent service extension. (#25977)

* Add didSendFirstFrameEvent service extension.

packages/flutter/lib/src/widgets/binding.dart

@@ -289,6 +289,15 @@ mixin WidgetsBinding on BindingBase, SchedulerBinding, GestureBinding, RendererB
           return _forceRebuild();
         },
       );
+
+      registerServiceExtension(
+        name: 'didSendFirstFrameEvent',
+        callback: (_) async {
+          return <String, dynamic>{
+            'enabled': _needToReportFirstFrame ? 'false' : 'true'
+          };
+        },
+      );
     }
 
     assert(() {

packages/flutter/test/foundation/service_extensions_test.dart

@@ -104,7 +104,20 @@ void main() {
   test('Service extensions - pretest', () async {
     binding = TestServiceExtensionsBinding();
     expect(binding.frameScheduled, isTrue);
+
+    // We need to test this service extension here because the result is true
+    // after the first binding.doFrame() call.
+    Map<String, dynamic> firstFrameResult;
+    expect(binding.debugDidSendFirstFrameEvent, isFalse);
+    firstFrameResult = await binding.testExtension('didSendFirstFrameEvent', <String, String>{});
+    expect(firstFrameResult, <String, String>{ 'enabled': 'false' });
+
     await binding.doFrame(); // initial frame scheduled by creating the binding
+
+    expect(binding.debugDidSendFirstFrameEvent, isTrue);
+    firstFrameResult = await binding.testExtension('didSendFirstFrameEvent', <String, String>{});
+    expect(firstFrameResult, <String, String>{ 'enabled': 'true' });
+
     expect(binding.frameScheduled, isFalse);
 
     expect(debugPrint, equals(debugPrintThrottled));
@@ -548,7 +561,7 @@ void main() {
 
     // If you add a service extension... TEST IT! :-)
     // ...then increment this number.
-    expect(binding.extensions.length, 24 + widgetInspectorExtensionCount);
+    expect(binding.extensions.length, 25 + widgetInspectorExtensionCount);
 
     expect(console, isEmpty);
     debugPrint = debugPrintThrottled;