Re-apply "Add assert that the root widget has been attached" (#33084)

This re-applies #32437 with a fix to the broken device lab test.

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

@@ -736,6 +736,12 @@ mixin WidgetsBinding on BindingBase, SchedulerBinding, GestureBinding, RendererB
     ).attachToRenderTree(buildOwner, renderViewElement);
   }
 
+  /// Whether the [renderViewElement] has been initialized.
+  ///
+  /// This will be false until [runApp] is called (or [WidgetTester.pumpWidget]
+  /// is called in the context of a [TestWidgetsFlutterBinding]).
+  bool get isRootWidgetAttached => _renderViewElement != null;
+
   @override
   Future<void> performReassemble() {
     assert(() {

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

@@ -15,6 +15,9 @@ class GetHealth extends Command {
 
   @override
   String get kind => 'get_health';
+
+  @override
+  bool get requiresRootWidgetAttached => false;
 }
 
 /// A description of application state.

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

@@ -32,6 +32,20 @@ abstract class Command {
   /// Identifies the type of the command object and of the handler.
   String get kind;
 
+  /// 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;
+
   /// Serializes this command to parameter name/value pairs.
   @mustCallSuper
   Map<String, String> serialize() {

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

@@ -21,6 +21,9 @@ class RequestData extends Command {
   @override
   String get kind => 'request_data';
 
+  @override
+  bool get requiresRootWidgetAttached => false;
+
   @override
   Map<String, String> serialize() => super.serialize()..addAll(<String, String>{
     'message': message,

packages/flutter_driver/lib/src/extension/extension.dart

@@ -183,6 +183,8 @@ class FlutterDriverExtension {
       if (commandHandler == null || commandDeserializer == null)
         throw 'Extension $_extensionMethod does not support command $commandKind';
       final Command command = commandDeserializer(params);
+      assert(WidgetsBinding.instance.isRootWidgetAttached || !command.requiresRootWidgetAttached,
+          'No root widget is attached; have you remembered to call runApp()?');
       Future<Result> responseFuture = commandHandler(command);
       if (command.timeout != null)
         responseFuture = responseFuture.timeout(command.timeout);