Support multiple serialization sources in DeserializerSystemImpl.

pkg/compiler/lib/src/serialization/task.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.task;
 
 import 'dart:async' show EventSink, Future;
 import '../common.dart';
 import '../common/resolution.dart' show ResolutionImpact, ResolutionWorkItem;
 import '../common/tasks.dart' show CompilerTask;
@@ skipping 94 matching lines @@
     measure(() {
       sink
         ..add(createSerializer(libraries)
             .toText(const JsonSerializationEncoder()))
         ..close();
     });
   }
 
   void deserializeFromText(String serializedData) {
     measure(() {
+      if (deserializer == null) {
+        deserializer = new DeserializerSystemImpl(
+            compiler, compiler.backend.impactTransformer);
+      }
+      DeserializerSystemImpl deserializerImpl = deserializer;
+      DeserializationContext context = deserializerImpl.deserializationContext;
       Deserializer dataDeserializer = new Deserializer.fromText(
-          new DeserializationContext(),
-          serializedData,
-          const JsonSerializationDecoder());
-      dataDeserializer.plugins.add(compiler.backend.serialization.deserializer);
-      deserializer = new DeserializerSystemImpl(
-          compiler, dataDeserializer, compiler.backend.impactTransformer);
+          context, serializedData, const JsonSerializationDecoder());
+      context.deserializers.add(dataDeserializer);
     });
   }
 }
 
 /// A [ResolutionWorkItem] for a deserialized element.
 ///
 /// This will not resolve the element but only compute the [WorldImpact].
 class DeserializedResolutionWorkItem implements ResolutionWorkItem {
   final Element element;
   final ItemCompilationContext compilationContext;
@@ skipping 18 matching lines @@
 /// elements.
 abstract class DeserializerSystem {
   Future<LibraryElement> readLibrary(Uri resolvedUri);
   bool isDeserialized(Element element);
   bool hasResolvedAst(ExecutableElement element);
   ResolvedAst getResolvedAst(ExecutableElement element);
   bool hasResolutionImpact(Element element);
   ResolutionImpact getResolutionImpact(Element element);
   WorldImpact computeWorldImpact(Element element);
 }