Move ElementEnvironment.isSubtype to DartTypes

pkg/compiler/lib/src/frontend_strategy.dart

Index: pkg/compiler/lib/src/frontend_strategy.dart
diff --git a/pkg/compiler/lib/src/frontend_strategy.dart b/pkg/compiler/lib/src/frontend_strategy.dart
index 80f4bd22aac3fac7b10fa1b83004c0bbca664e56..a96ab6f5eba98ddc8b81b4a40b986ea9b3312161 100644
--- a/pkg/compiler/lib/src/frontend_strategy.dart
+++ b/pkg/compiler/lib/src/frontend_strategy.dart
@@ -9,6 +9,7 @@ import 'common_elements.dart';
 import 'common/backend_api.dart';
 import 'common/tasks.dart';
 import 'elements/entities.dart';
+import 'elements/types.dart';
 import 'environment.dart';
 import 'enqueue.dart';
 import 'js_backend/backend.dart';
@@ -46,6 +47,9 @@ abstract class FrontEndStrategy {
   /// strategy.
   ElementEnvironment get elementEnvironment;
 
+  /// Returns the [DartTypes] for the element model used in this strategy.
+  DartTypes get dartTypes;
+
   /// Returns the [AnnotationProcessor] for this strategy.
   AnnotationProcessor get annotationProcesser;