Access ConstantSystem through ClosedWorld.

pkg/compiler/lib/src/world.dart

Index: pkg/compiler/lib/src/world.dart
diff --git a/pkg/compiler/lib/src/world.dart b/pkg/compiler/lib/src/world.dart
index 3de6d0f6c9df6077c1e3dfeb3f7e98dcfc578e53..76b2bc3e44d52e20b88a68ea27fcddf0b039895c 100644
--- a/pkg/compiler/lib/src/world.dart
+++ b/pkg/compiler/lib/src/world.dart
@@ -8,6 +8,7 @@ import 'cache_strategy.dart';
 import 'closure.dart' show SynthesizedCallMethodElementX;
 import 'common/backend_api.dart' show BackendClasses;
 import 'common.dart';
+import 'constants/constant_system.dart';
 import 'core_types.dart' show CoreTypes, CoreClasses, CommonElements;
 import 'dart_types.dart';
 import 'elements/elements.dart'
@@ -53,6 +54,8 @@ abstract class ClosedWorld implements World {
 
   CommonMasks get commonMasks;
 
+  ConstantSystem get constantSystem;
+
   /// Returns `true` if [cls] is either directly or indirectly instantiated.
   bool isInstantiated(ClassElement cls);
 
@@ -908,6 +911,8 @@ class WorldImpl implements ClosedWorld, ClosedWorldRefiner, OpenWorld {
 
   CoreClasses get coreClasses => commonElements;
 
+  ConstantSystem get constantSystem => _backend.constantSystem;
+
   /// Called to add [cls] to the set of known classes.
   ///
   /// This ensures that class hierarchy queries can be performed on [cls] and