Support serialization of WorldImpact

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 551ba499106660dbac20abafede50f78c6cb6f7e..fc1dfaebfeb1a861fdae94f000c790c6ba0e553b 100644
--- a/pkg/compiler/lib/src/serialization/task.dart
+++ b/pkg/compiler/lib/src/serialization/task.dart
@@ -26,6 +26,11 @@ class SerializationTask extends CompilerTask {
 
   String get name => 'Serialization';
 
+  /// If `true`, data must be retained to support serialization.
+  // TODO(johnniwinther): Make this more precise in terms of what needs to be
+  // retained, for instance impacts, resolution data etc.
+  bool supportSerialization = false;
+
   /// Returns the [LibraryElement] for [resolvedUri] if available from
   /// serialization.
   LibraryElement readLibrary(Uri resolvedUri) {