Support multiple serialization sources in DeserializerSystemImpl.

pkg/compiler/lib/src/serialization/task.dart

Index: pkg/compiler/lib/src/serialization/task.dart
diff --git a/pkg/compiler/lib/src/serialization/task.dart b/pkg/compiler/lib/src/serialization/task.dart
index c09c0e50f62b4d607da7f1f4b4e6d1331ad1f0a9..21618595db1e61e99c7cdfb5f986c9d77b79f6c1 100644
--- a/pkg/compiler/lib/src/serialization/task.dart
+++ b/pkg/compiler/lib/src/serialization/task.dart
@@ -112,13 +112,15 @@ class SerializationTask extends CompilerTask implements LibraryDeserializer {
 
   void deserializeFromText(String serializedData) {
     measure(() {
+      if (deserializer == null) {
+        deserializer = new DeserializerSystemImpl(
+            compiler, compiler.backend.impactTransformer);
+      }
+      DeserializerSystemImpl deserializerImpl = deserializer;
       Deserializer dataDeserializer = new Deserializer.fromText(
-          new DeserializationContext(),
+          deserializerImpl.deserializationContext,
           serializedData,
           const JsonSerializationDecoder());
-      dataDeserializer.plugins.add(compiler.backend.serialization.deserializer);
-      deserializer = new DeserializerSystemImpl(
-          compiler, dataDeserializer, compiler.backend.impactTransformer);
     });
   }
 }