Unify private names treatment.

lib/compiler/implementation/dart_backend/placeholder_collector.dart

Index: lib/compiler/implementation/dart_backend/placeholder_collector.dart
diff --git a/lib/compiler/implementation/dart_backend/placeholder_collector.dart b/lib/compiler/implementation/dart_backend/placeholder_collector.dart
index ac3a8bff96ca1d805fd015f94c1e13e2cecabaa0..d7ed5d5af021d5d4b19958c92efe7be021e8650c 100644
--- a/lib/compiler/implementation/dart_backend/placeholder_collector.dart
+++ b/lib/compiler/implementation/dart_backend/placeholder_collector.dart
@@ -34,7 +34,7 @@ class SendVisitor extends ResolvedVisitor {
   visitGetterSend(Send node) {
     final element = elements[node];
     // element === null means dynamic property access.
-    if (element === null || element.isInstanceMember()) {
+    if (element === null || element.isMember()) {
       tryRenamePrivateSelector(node);
       return;
     }
@@ -296,11 +296,12 @@ class PlaceholderCollector extends AbstractVisitor {
   }
 
   visitSendSet(SendSet send) {
+    if (send.selector is Identifier) {
+      tryMakePrivateIdentifier(send.selector.asIdentifier());
+    }
     final element = treeElements[send];
     if (element !== null) {
-      if (element.isInstanceMember()) {
-        tryMakePrivateIdentifier(send.selector.asIdentifier());
-      } else if (element.isTopLevel()) {
+      if (element.isTopLevel()) {
         assert(element is VariableElement || element.isSetter());
         makeElementPlaceholder(send.selector, element);
       } else {