Fix hints reported by analyzer.

pkg/compiler/lib/src/serialization/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;
 
 import '../common.dart';
 import '../common/resolution.dart';
 import '../constants/expressions.dart';
 import '../elements/resolution_types.dart';
@@ skipping 641 matching lines @@
 ///         {...}, // [ObjectValue] of the 0th [ConstantExpression].
 ///         ...
 ///         {...}, // [ObjectValue] of the n-th [ConstantExpression].
 ///       ],
 ///     }
 ///
 // TODO(johnniwinther): Support dependencies between serialized subcomponent.
 class Serializer {
   List<SerializerPlugin> plugins = <SerializerPlugin>[];
 
-  Map<Uri, dynamic> _dependencyMap = <Uri, dynamic>{};
   Map<Element, DataObject> _elementMap = <Element, DataObject>{};
   Map<ConstantExpression, DataObject> _constantMap =
       <ConstantExpression, DataObject>{};
   Map<ResolutionDartType, DataObject> _typeMap =
       <ResolutionDartType, DataObject>{};
   List _pendingList = [];
   ElementMatcher shouldInclude;
 
   // TODO(johnniwinther): Replace [includeElement] with a general strategy.
   Serializer({this.shouldInclude: includeAllElements});
@@ skipping 467 matching lines @@
 
   /// Returns the value used to store [key] as a property in the encoding an
   /// [ObjectValue].
   ///
   /// Different encodings have different restrictions and capabilities as how
   /// to store a [Key] value. For instance: A JSON encoding needs to convert
   /// [Key] to a [String] to store it in a JSON object; a Dart encoding can
   /// choose to store a [Key] as an [int] or as the [Key] itself.
   getObjectPropertyValue(Key key);
 }