Use Entity in native_data.

pkg/compiler/lib/src/kernel/world_builder.dart

Index: pkg/compiler/lib/src/kernel/world_builder.dart
diff --git a/pkg/compiler/lib/src/kernel/world_builder.dart b/pkg/compiler/lib/src/kernel/world_builder.dart
index 615384888785c5d9e46be242ae4745625ef94186..bb6b5fd402204e926f9c094ec0fae19dfda29850 100644
--- a/pkg/compiler/lib/src/kernel/world_builder.dart
+++ b/pkg/compiler/lib/src/kernel/world_builder.dart
@@ -565,6 +565,11 @@ class KernelElementEnvironment implements ElementEnvironment {
   }
 
   @override
+  ClassEntity getSuperClass(ClassEntity cls) {
+    throw new UnimplementedError('KernelElementEnvironment.getSuperClass');
+  }
+
+  @override
   void forEachMixin(ClassEntity cls, void f(ClassEntity mixin)) {
     throw new UnimplementedError('KernelElementEnvironment.forEachMixin');
   }