Use entities in NativeResolutionEnqueuer and make it reusable for kernel

pkg/compiler/lib/src/common_elements.dart

Index: pkg/compiler/lib/src/common_elements.dart
diff --git a/pkg/compiler/lib/src/common_elements.dart b/pkg/compiler/lib/src/common_elements.dart
index 4c1143342b995dfda74d8993593af2fb1946289c..000cf0eb8ebeca8bf870cd9c3a13fe0a626a98e0 100644
--- a/pkg/compiler/lib/src/common_elements.dart
+++ b/pkg/compiler/lib/src/common_elements.dart
@@ -282,6 +282,9 @@ abstract class ElementEnvironment {
   /// Returns the 'this type' of [cls]. That is, the instantiation of [cls]
   /// where the type arguments are the type variables of [cls].
   InterfaceType getThisType(ClassEntity cls);
+
+  /// Returns `true` if [a] is a subtype of [b].
+  bool isSubtype(DartType a, DartType b);
 }
 
 class CommonElementsImpl implements CommonElements {