Create closed world for hello world using .dill file

pkg/compiler/lib/src/kernel/kernel_strategy.dart

Index: pkg/compiler/lib/src/kernel/kernel_strategy.dart
diff --git a/pkg/compiler/lib/src/kernel/kernel_strategy.dart b/pkg/compiler/lib/src/kernel/kernel_strategy.dart
index c32b756e300b818c3bca2c733ec8d5eef9b81816..24a0fc414f33dfa8a2f3d6b7dccf44b94733b569 100644
--- a/pkg/compiler/lib/src/kernel/kernel_strategy.dart
+++ b/pkg/compiler/lib/src/kernel/kernel_strategy.dart
@@ -131,6 +131,12 @@ class KernelFrontEndStrategy implements FrontEndStrategy {
     return new KernelWorkItemBuilder(
         elementMap, nativeBasicData, nativeDataBuilder, impactTransformer);
   }
+
+  @override
+  SourceSpan spanFromSpannable(Spannable spannable, Entity currentElement) {
+    // TODO(johnniwinther): Compute source spans from kernel elements.
+    return new SourceSpan(null, null, null);
+  }
 }
 
 class KernelWorkItemBuilder implements WorkItemBuilder {