Move ServiceMocker into flutter_test

People writing their own tests will want to mock services as well.

packages/flutter/test/widget/input_test.dart

@@ -8,8 +8,6 @@ import 'package:flutter/rendering.dart';
 import 'package:sky_services/editing/editing.mojom.dart' as mojom;
 import 'package:test/test.dart';
 
-import '../services/mock_services.dart';
-
 class MockKeyboard implements mojom.Keyboard {
   mojom.KeyboardClient client;
 

packages/flutter_test/lib/flutter_test.dart

@@ -5,5 +5,6 @@
 /// Testing library for flutter, built on top of package:test.
 library flutter_test;
 
+export 'src/service_mocker.dart';
 export 'src/test_pointer.dart';
 export 'src/widget_tester.dart';