Allow waitUntilFirstFrameRasterized without a root widget (#56430)

packages/flutter_driver/lib/src/common/wait.dart

@@ -31,6 +31,9 @@ class WaitForCondition extends Command {
 
   @override
   String get kind => 'waitForCondition';
+
+  @override
+  bool get requiresRootWidgetAttached => condition.requiresRootWidgetAttached;
 }
 
 /// A Flutter Driver command that waits until there are no more transient callbacks in the queue.
@@ -148,6 +151,20 @@ abstract class SerializableWaitCondition {
       'conditionName': conditionName
     };
   }
+
+  /// Whether this command requires the widget tree to be initialized before
+  /// the command may be run.
+  ///
+  /// This defaults to true to force the application under test to call [runApp]
+  /// before attempting to remotely drive the application. Subclasses may
+  /// override this to return false if they allow invocation before the
+  /// application has started.
+  ///
+  /// See also:
+  ///
+  ///  * [WidgetsBinding.isRootWidgetAttached], which indicates whether the
+  ///    widget tree has been initialized.
+  bool get requiresRootWidgetAttached => true;
 }
 
 /// A condition that waits until no transient callbacks are scheduled.
@@ -208,6 +225,9 @@ class FirstFrameRasterized extends SerializableWaitCondition {
 
   @override
   String get conditionName => 'FirstFrameRasterizedCondition';
+
+  @override
+  bool get requiresRootWidgetAttached => false;
 }
 
 /// A condition that waits until there are no pending platform messages.

packages/flutter_driver/test/src/real_tests/wait_test.dart

@@ -41,6 +41,17 @@ void main() {
       expect(waitForCondition.condition, equals(const NoTransientCallbacks()));
       expect(waitForCondition.timeout, equals(const Duration(milliseconds: 10)));
     });
+
+    test('WaitForCondition requiresRootWidget', () {
+        expect(
+            const WaitForCondition(NoTransientCallbacks())
+                .requiresRootWidgetAttached,
+            isTrue);
+        expect(
+            const WaitForCondition(FirstFrameRasterized())
+                .requiresRootWidgetAttached,
+            isFalse);
+      });
   });
 
   group('NoTransientCallbacksCondition', () {
@@ -113,6 +124,10 @@ void main() {
           throwsA(predicate<SerializationException>((SerializationException e) =>
               e.message == 'Error occurred during deserializing the FirstFrameRasterizedCondition JSON string: {conditionName: Unknown}')));
     });
+
+    test('FirstFrameRasterizedCondition requiresRootWidget', () {
+      expect(const FirstFrameRasterized().requiresRootWidgetAttached, isFalse);
+    });
   });
 
   group('CombinedCondition', () {