Re-add annotations to test files that use null safety features

lib/ui/fixtures/ui_test.dart

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart = 2.10
+
 import 'dart:async';
 import 'dart:typed_data';
 import 'dart:ui';

runtime/fixtures/runtime_test.dart

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart = 2.10
+
 import 'dart:async';
 import 'dart:isolate';
 import 'dart:ui';

runtime/fixtures/split_lib_test.dart

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart = 2.10
+
 library splitlib;
 
 int splitAdd(int i, int j) {

shell/common/fixtures/shell_test.dart

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart = 2.10
+
 import 'dart:convert' show utf8, json;
 import 'dart:isolate';
 import 'dart:typed_data';

shell/platform/embedder/fixtures/main.dart

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart = 2.10
+
 import 'dart:async';
 import 'dart:typed_data';
 import 'dart:ui';