Split loader from the rest of the compiler. This adds several abstractions to

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

Index: pkg/compiler/lib/src/serialization/serialization.dart
diff --git a/pkg/compiler/lib/src/serialization/serialization.dart b/pkg/compiler/lib/src/serialization/serialization.dart
index 0379b6ad5fd67fc196b40c822198a3a11c436157..967d2ee6b71c7868d11a2ea7d869cf93144482a2 100644
--- a/pkg/compiler/lib/src/serialization/serialization.dart
+++ b/pkg/compiler/lib/src/serialization/serialization.dart
@@ -18,6 +18,8 @@ import 'keys.dart';
 import 'type_serialization.dart';
 import 'values.dart';
 
+export 'task.dart' show LibraryDeserializer;
+
 /// An object that supports the encoding an [ObjectValue] for serialization.
 ///
 /// The [ObjectEncoder] ensures that nominality and circularities of