Make the ClosedWorld build the closure class on the kernel side.

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 6ed79c20dfc0967ba4141330914e1144d62fdbca..e7fa466fc4c339209a206e5b3ea3938ab2ec6252 100644
--- a/pkg/compiler/lib/src/js_model/elements.dart
+++ b/pkg/compiler/lib/src/js_model/elements.dart
@@ -314,24 +314,16 @@ class JClass implements ClassEntity, IndexedClass {
 
 abstract class JMember implements MemberEntity, IndexedMember {
   /// Member index used for fast lookup in [JsToFrontendMapImpl].
-  int _memberIndex;
+  final 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;