Eliminate multi-callback structure for LibraryLoader.

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 68f5f793239413cfad6a077afdf19151c120da91..6df97636dbbf9aeafc6f6e769a483ac6ed4ba3f2 100644
--- a/pkg/compiler/lib/src/serialization/task.dart
+++ b/pkg/compiler/lib/src/serialization/task.dart
@@ -22,6 +22,9 @@ abstract class LibraryDeserializer {
   /// Loads the [LibraryElement] associated with a library under [uri], or null
   /// if no serialized information is available for the given library.
   Future<LibraryElement> readLibrary(Uri uri);
+
+  /// Returns `true` if [element] has been deserialized.
+  bool isDeserialized(Element element);
 }
 
 /// Task that supports deserialization of elements.