Create closed world for hello world using .dill file

pkg/compiler/lib/src/frontend_strategy.dart

Index: pkg/compiler/lib/src/frontend_strategy.dart
diff --git a/pkg/compiler/lib/src/frontend_strategy.dart b/pkg/compiler/lib/src/frontend_strategy.dart
index 575e075159e1dcd2580f798ab9eb6714cf9e536e..5ecab116241efbe1f62127f2f128613e1a0d2218 100644
--- a/pkg/compiler/lib/src/frontend_strategy.dart
+++ b/pkg/compiler/lib/src/frontend_strategy.dart
@@ -97,6 +97,9 @@ abstract class FrontEndStrategy {
 
   /// Creates the [RuntimeTypesNeedBuilder] for this strategy.
   RuntimeTypesNeedBuilder createRuntimeTypesNeedBuilder();
+
+  /// Creates a [SourceSpan] from [spannable] in context of [currentElement].
+  SourceSpan spanFromSpannable(Spannable spannable, Entity currentElement);
 }
 
 /// Class that performs the mechanics to investigate annotations in the code.