Register created closure class with the world.

pkg/compiler/lib/src/js_model/elements.dart

Index: pkg/compiler/lib/src/js_model/elements.dart
diff --git a/pkg/compiler/lib/src/js_model/elements.dart b/pkg/compiler/lib/src/js_model/elements.dart
index b040bc03530df2897945eacc73f8059d00381e65..1d02a1aa45a59a113f88bc04250c3e86298b259e 100644
--- a/pkg/compiler/lib/src/js_model/elements.dart
+++ b/pkg/compiler/lib/src/js_model/elements.dart
@@ -323,16 +323,24 @@ class JClass implements ClassEntity, IndexedClass {
 
 abstract class JMember implements MemberEntity, IndexedMember {
   /// Member index used for fast lookup in [JsToFrontendMapImpl].
-  final int memberIndex;
+  int _memberIndex;
   final JLibrary library;
   final JClass enclosingClass;
   final Name _name;
   final bool _isStatic;
 
-  JMember(this.memberIndex, this.library, this.enclosingClass, this._name,
+  JMember(this._memberIndex, this.library, this.enclosingClass, this._name,
       {bool isStatic: false})
       : _isStatic = isStatic;
 
+  int get memberIndex => _memberIndex;
+
+  /// Should only be called by closure methods. All others should set
+  /// memberIndex at initialization time.
+  void set setClosureMemberIndex(int newIndex) {
+    _memberIndex = newIndex;
+  }
+
   String get name => _name.text;
 
   Name get memberName => _name;