Move ElementEnvironment.isSubtype to DartTypes

pkg/compiler/lib/src/resolution/resolution_strategy.dart

Index: pkg/compiler/lib/src/resolution/resolution_strategy.dart
diff --git a/pkg/compiler/lib/src/resolution/resolution_strategy.dart b/pkg/compiler/lib/src/resolution/resolution_strategy.dart
index a8616f48ff3313cd481a3d5b2db109446d9c276b..db2a54de2fbf590f04e45a94399c5731410bfa7a 100644
--- a/pkg/compiler/lib/src/resolution/resolution_strategy.dart
+++ b/pkg/compiler/lib/src/resolution/resolution_strategy.dart
@@ -17,6 +17,7 @@ import '../elements/elements.dart';
 import '../elements/entities.dart';
 import '../elements/modelx.dart';
 import '../elements/resolution_types.dart';
+import '../elements/types.dart';
 import '../environment.dart';
 import '../enqueue.dart';
 import '../frontend_strategy.dart';
@@ -49,6 +50,8 @@ class ResolutionFrontEndStrategy implements FrontEndStrategy {
   ResolutionFrontEndStrategy(this._compiler)
       : elementEnvironment = new _CompilerElementEnvironment(_compiler);
 
+  DartTypes get dartTypes => _compiler.types;
+
   LibraryLoaderTask createLibraryLoader(
       ResolvedUriTranslator uriTranslator,
       ScriptLoader scriptLoader,
@@ -257,11 +260,6 @@ class _CompilerElementEnvironment implements ElementEnvironment {
   }
 
   @override
-  bool isSubtype(ResolutionDartType a, ResolutionDartType b) {
-    return _compiler.types.isSubtype(a, b);
-  }
-
-  @override
   MemberElement lookupClassMember(ClassElement cls, String name,
       {bool setter: false, bool required: false}) {
     cls.ensureResolved(_resolution);