Avoid duplicate effort in _getElementReferenceId

pkg/analyzer/lib/src/summary/summarize_elements.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 serialization.elements;
 
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/dart/element/type.dart';
 import 'package:analyzer/src/dart/element/element.dart';
 import 'package:analyzer/src/dart/element/member.dart';
@@ skipping 973 matching lines @@
     int slot = ++numSlots;
     if (type != null) {
       deferredLinkedTypes
           .add(() => serializeTypeRef(type, context, linked: true, slot: slot));
     }
     return slot;
   }
 
   int _getElementReferenceId(Element element, {bool linked: false}) {
     return referenceMap.putIfAbsent(element, () {
-      LibraryElement dependentLibrary;
+      LibraryElement dependentLibrary = librarySerializer.libraryElement;
+      int unit = 0;
       if (element != null) {
         Element enclosingElement = element.enclosingElement;
         if (enclosingElement is CompilationUnitElement) {
           dependentLibrary = enclosingElement.library;
+          unit = dependentLibrary.units.indexOf(enclosingElement);
+          assert(unit != -1);
         }
       }
-      int unit;
-      if (dependentLibrary == null) {
-        unit = 0;
-        dependentLibrary = librarySerializer.libraryElement;
-      } else {
-        CompilationUnitElement unitElement =
-            element.getAncestor((Element e) => e is CompilationUnitElement);
-        unit = dependentLibrary.units.indexOf(unitElement);
-        assert(unit != -1);
-      }
       ReferenceKind kind = _getReferenceKind(element);
       String name = element == null ? 'void' : element.name;
       int index;
       LinkedReferenceBuilder linkedReference;
       if (linked) {
         linkedReference =
             new LinkedReferenceBuilder(kind: kind, unit: unit, name: name);
         Element enclosing = element?.enclosingElement;
         if (enclosing is ClassElement) {
           linkedReference.containingReference =
@@ skipping 369 matching lines @@
       exportNames.add(new LinkedExportNameBuilder(
           name: name,
           dependency: serializeDependency(dependentLibrary),
           unit: unit,
           kind: kind));
     }
     pb.exportNames = exportNames;
     return pb;
   }
 }