Declare IndexableObject.file instead of Source.

pkg/analysis_server/lib/src/services/index/indexable_element.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 src.services.index;
 
 import 'dart:collection';
 
 import 'package:analysis_server/plugin/index/index_core.dart';
 import 'package:analyzer/src/generated/element.dart';
@@ skipping 27 matching lines @@
 
   @override
   int get offset {
     if (element is ConstructorElement) {
       return element.enclosingElement.nameOffset;
     }
     return element.nameOffset;
   }
 
   @override
-  Source get source => element.source;
+  String get filePath {
+    return element.source?.fullName;
+  }
 
   @override
   bool operator ==(Object object) =>
       object is IndexableElement && element == object.element;
 
   @override
   String toString() => element.toString();
 }
 
 /**
@@ skipping 105 matching lines @@
       ClassElement classElement = element.enclosingElement;
       int constructorIndex = classElement.constructors.indexOf(element);
       return _constructorKinds.putIfAbsent(constructorIndex,
           () => new IndexableElementKind._(ElementKind.CONSTRUCTOR));
     }
     ElementKind elementKind = element.kind;
     return _kindMap.putIfAbsent(
         elementKind, () => new IndexableElementKind._(elementKind));
   }
 }