Add strong mode type checking pass.

test/baseline_type_propagation_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/text/ast_to_text.dart';
 import 'package:kernel/transformations/mixin_full_resolution.dart';
 import 'package:kernel/type_propagation/builder.dart';
 import 'package:kernel/type_propagation/solver.dart';
 import 'package:kernel/type_propagation/visualizer.dart';
 
 import 'baseline_tester.dart';
 
 class TypePropagationTest extends TestTarget {
   @override
   Annotator annotator;
 
   @override
   List<String> get extraRequiredLibraries => [];
 
   @override
   String get name => 'type-propagation-test';
 
   @override
   bool get strongMode => false;
 
   @override
-  void transformProgram(Program program) {
+  List<String> transformProgram(Program program) {
     new MixinFullResolution().transform(program);
     var visualizer = new Visualizer(program);
     var builder = new Builder(program, visualizer: visualizer);
     var solver = new Solver(builder);
     solver.solve();
     visualizer.solver = solver;
     annotator = new TextAnnotator(visualizer);
+    return const <String>[];
   }
 }
 
 void main() {
   runBaselineTests('type-propagation', new TypePropagationTest());
 }