Add strong mode type checking pass.

test/baseline_strong_mode_test.dart

Index: test/baseline_strong_mode_test.dart
diff --git a/test/baseline_strong_mode_test.dart b/test/baseline_strong_mode_test.dart
index fa8626911fe3713fab29ee970c4d8bfe1c6ce8ac..51d2a7c032e49f1b35cb1548a97e34d8ad16328f 100644
--- a/test/baseline_strong_mode_test.dart
+++ b/test/baseline_strong_mode_test.dart
@@ -1,8 +1,12 @@
 // 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/class_hierarchy.dart';
+import 'package:kernel/core_types.dart';
 import 'package:kernel/kernel.dart';
 import 'package:kernel/transformations/mixin_full_resolution.dart';
+import 'package:kernel/type_checker.dart';
+import 'package:path/path.dart' as pathlib;
 
 import 'baseline_tester.dart';
 
@@ -17,8 +21,40 @@ class StrongModeTest extends TestTarget {
   bool get strongMode => true;
 
   @override
-  void transformProgram(Program program) {
+  List<String> transformProgram(Program program) {
+    List<String> errors = <String>[];
     new MixinFullResolution().transform(program);
+    new TestTypeChecker(
+            errors, new CoreTypes(program), new ClassHierarchy(program))
+        .checkProgram(program);
+    return errors;
+  }
+}
+
+class TestTypeChecker extends TypeChecker {
+  final List<String> errors;
+
+  TestTypeChecker(this.errors, CoreTypes coreTypes, ClassHierarchy hierarchy)
+      : super(coreTypes, hierarchy);
+
+  @override
+  void checkAssignable(TreeNode where, DartType from, DartType to) {
+    if (!environment.isSubtypeOf(from, to)) {
+      fail(where, '$from is not a subtype of $to');
+    }
+  }
+
+  @override
+  void fail(TreeNode where, String message) {
+    var location = where.location;
+    var locationString;
+    if (location != null) {
+      var file = pathlib.basename(Uri.parse(location.file).path);
+      locationString = '($file:${location.line}:${location.column})';
+    } else {
+      locationString = '(no location)';
+    }
+    errors.add('$message $locationString');
   }
 }