Add strong mode type checking pass.

test/baseline_spec_mode_test.dart

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 import 'package:kernel/kernel.dart';
 import 'package:kernel/transformations/mixin_full_resolution.dart';
 
 import 'baseline_tester.dart';
 
 class SpecModeTest extends TestTarget {
   @override
   List<String> get extraRequiredLibraries => [];
 
   @override
   String get name => 'spec-mode-test';
 
   @override
   bool get strongMode => false;
 
   @override
-  void transformProgram(Program program) {
+  List<String> transformProgram(Program program) {
     new MixinFullResolution().transform(program);
+    return const <String>[];
   }
 }
 
 void main() {
   runBaselineTests('spec-mode', new SpecModeTest());
 }