Make .enclosingClass always return the declaration

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

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library dart2js.serialization.elements;
 
 import '../common.dart';
 import '../constants/constructors.dart';
 import '../constants/expressions.dart';
 import '../elements/resolution_types.dart';
@@ skipping 148 matching lines @@
       list ??= encoder.createList(Key.METADATA);
       element.implementation.metadata.forEach(encodeAnnotation);
     }
   }
 
   /// Serialize the parent relation for [element] into [encoder], i.e library,
   /// enclosing class, and compilation unit references.
   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 {
       encoder.setElement(Key.LIBRARY, element.library);
       encoder.setElement(Key.COMPILATION_UNIT, element.compilationUnit);
     }
   }
 
   /// Serialize the parameters of [element] into [encoder].
   static void serializeParameters(
@@ skipping 769 matching lines @@
             enclosingElement, existingElement, newElement);
       case SerializedElementKind.EXTERNAL_LIBRARY:
       case SerializedElementKind.EXTERNAL_LIBRARY_MEMBER:
       case SerializedElementKind.EXTERNAL_CLASS_MEMBER:
       case SerializedElementKind.EXTERNAL_CONSTRUCTOR:
         break;
     }
     throw new UnsupportedError("Unexpected element kind '${elementKind}.");
   }
 }