Make .enclosingClass always return the declaration

pkg/compiler/lib/src/serialization/element_serialization.dart

Index: pkg/compiler/lib/src/serialization/element_serialization.dart
diff --git a/pkg/compiler/lib/src/serialization/element_serialization.dart b/pkg/compiler/lib/src/serialization/element_serialization.dart
index 743952ec07d050d82b8cc57c6e1c013a6876c785..0e5c562fc1340392372c47992c9b34b830bbd65b 100644
--- a/pkg/compiler/lib/src/serialization/element_serialization.dart
+++ b/pkg/compiler/lib/src/serialization/element_serialization.dart
@@ -166,8 +166,7 @@ class SerializerUtil {
   static void serializeParentRelation(Element element, ObjectEncoder encoder) {
     if (element.enclosingClass != null) {
       encoder.setElement(Key.CLASS, element.enclosingClass);
-      if (element.enclosingClass.declaration.compilationUnit !=
-          element.compilationUnit) {
+      if (element.enclosingClass.compilationUnit != element.compilationUnit) {
         encoder.setElement(Key.COMPILATION_UNIT, element.compilationUnit);
       }
     } else {