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 22c848f68abe0c767238f5fb779336c8d0df1ce7..f1e4fa5c1d2039b0f70836456bae0c5a38e947f2 100644
--- a/pkg/compiler/lib/src/frontend_strategy.dart
+++ b/pkg/compiler/lib/src/frontend_strategy.dart
@@ -87,6 +87,9 @@ 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.