Make closedWorld a property of ClosedWorldRefiner.

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 490390dd259c26114827d020879b8c0fd2e09923..04f79bdb902cc4602d517907f57a38abce60f249 100644
--- a/pkg/compiler/lib/src/world.dart
+++ b/pkg/compiler/lib/src/world.dart
@@ -298,6 +298,9 @@ abstract class ClosedWorld implements World {
 /// Interface for computing side effects and uses of elements. This is used
 /// during type inference to compute the [ClosedWorld] for code generation.
 abstract class ClosedWorldRefiner {
+  /// The closed world being refined.
+  ClosedWorld get closedWorld;
+
   /// Registers the side [effects] of executing [element].
   void registerSideEffects(Element element, SideEffects effects);
 
@@ -416,6 +419,9 @@ class ClosedWorldImpl implements ClosedWorld, ClosedWorldRefiner {
     _allFunctions = functionSetBuilder.close(this);
   }
 
+  @override
+  ClosedWorld get closedWorld => this;
+
   /// Cache of [FlatTypeMask]s grouped by the 8 possible values of the
   /// `FlatTypeMask.flags` property.
   final List<Map<ClassElement, TypeMask>> _canonicalizedTypeMasks =