dart2js cps minor change: Add a comment and rename a parameter.

pkg/compiler/lib/src/cps_ir/type_propagation.dart

Index: pkg/compiler/lib/src/cps_ir/type_propagation.dart
diff --git a/pkg/compiler/lib/src/cps_ir/type_propagation.dart b/pkg/compiler/lib/src/cps_ir/type_propagation.dart
index c537e219d748dc1c64836c8da7605190620a1a13..281de983c52489ec3aeea6b6719a7c5dc5bbe491 100644
--- a/pkg/compiler/lib/src/cps_ir/type_propagation.dart
+++ b/pkg/compiler/lib/src/cps_ir/type_propagation.dart
@@ -1609,6 +1609,8 @@ class TransformingVisitor extends LeafVisitor {
       // Check if any of the possible targets depend on the extra receiver
       // argument. Mixins do this, and tear-offs always needs the extra receiver
       // argument because BoundClosure uses it for equality and hash code.
+      // TODO(15933): Make automatically generated property extraction
+      // closures work with the dummy receiver optimization.
       bool needsReceiver(Element target) {
         if (target is! FunctionElement) return false;
         FunctionElement function = target;