Mark testWidgets() and testUsingContext() with @isTest. (#17772)

packages/flutter_test/lib/src/widget_tester.dart

@@ -10,6 +10,7 @@ import 'package:flutter/material.dart';
 import 'package:flutter/rendering.dart';
 import 'package:flutter/scheduler.dart';
 import 'package:flutter/widgets.dart';
+import 'package:meta/meta.dart';
 import 'package:test/test.dart' as test_package;
 
 import 'all_elements.dart';
@@ -48,6 +49,7 @@ typedef Future<Null> WidgetTesterCallback(WidgetTester widgetTester);
 ///       expect(find.text('Success'), findsOneWidget);
 ///     });
 /// ```
+@isTest
 void testWidgets(String description, WidgetTesterCallback callback, {
   bool skip: false,
   test_package.Timeout timeout

packages/flutter_tools/test/src/context.dart

@@ -21,6 +21,7 @@ import 'package:flutter_tools/src/ios/simulators.dart';
 import 'package:flutter_tools/src/ios/xcodeproj.dart';
 import 'package:flutter_tools/src/usage.dart';
 import 'package:flutter_tools/src/version.dart';
+import 'package:meta/meta.dart';
 import 'package:mockito/mockito.dart';
 import 'package:quiver/time.dart';
 import 'package:test/test.dart';
@@ -37,6 +38,7 @@ MockDoctor get testDoctor => context[Doctor];
 
 typedef void ContextInitializer(AppContext testContext);
 
+@isTest
 void testUsingContext(String description, dynamic testMethod(), {
   Timeout timeout,
   Map<Type, Generator> overrides: const <Type, Generator>{},