Use entities in NativeResolutionEnqueuer and make it reusable for kernel

pkg/compiler/lib/src/compiler.dart

Index: pkg/compiler/lib/src/compiler.dart
diff --git a/pkg/compiler/lib/src/compiler.dart b/pkg/compiler/lib/src/compiler.dart
index 8147f241718a3fa7bb6c822e81241de1501e1ce8..437a7b145c0946d6cf46fb231d707120bd4f31cf 100644
--- a/pkg/compiler/lib/src/compiler.dart
+++ b/pkg/compiler/lib/src/compiler.dart
@@ -1841,6 +1841,11 @@ 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);