Fix dart2js warnings and add test to ensure it stays clean.

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 'package:front_end/src/fasta/scanner/precedence.dart'
     show PrecedenceInfo;
 
 import '../common.dart';
@@ skipping 921 matching lines @@
 class DeserializerPlugin {
   const DeserializerPlugin();
 
   /// Called upon the deserialization of [element].
   ///
   /// Use [getDecoder] to retrieve the data object with id [tag] stored for
   /// [element]. If not object is stored for [tag], [getDecoder] returns `null`.
   void onElement(Element element, ObjectDecoder getDecoder(String tag)) {}
 
   /// Called to deserialize custom data from [decoder].
-  dynamic onData(ObjectDecoder decoder) {}
+  dynamic onData(ObjectDecoder decoder) => null;
 }
 
 /// Context for parallel deserialization.
 class DeserializationContext {
   final DiagnosticReporter reporter;
   final Resolution resolution;
   final LibraryProvider libraryProvider;
   Map<Uri, LibraryElement> _uriMap = <Uri, LibraryElement>{};
   List<Deserializer> deserializers = <Deserializer>[];
   List<DeserializerPlugin> plugins = <DeserializerPlugin>[];
@@ skipping 199 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);
 }