A visitor that builds a map from node to "Usage".

lib/compiler/implementation/dart_backend/placeholder.dart

Index: lib/compiler/implementation/dart_backend/placeholder.dart
diff --git a/lib/compiler/implementation/dart_backend/placeholder.dart b/lib/compiler/implementation/dart_backend/placeholder.dart
new file mode 100644
index 0000000000000000000000000000000000000000..1b0545f7f639de10003907521893eae8044f6a00
--- /dev/null
+++ b/lib/compiler/implementation/dart_backend/placeholder.dart
@@ -0,0 +1,28 @@
+// Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+class Placeholder {
+  abstract String rename(ConflictingRenamer renamer);
+}
+
+class NullPlaceholder extends Placeholder {
+  String rename(ConflictingRenamer renamer) => '';
+  String toString() => 'null_placeholder[]';
+}
+
+class PrivatePlaceholder extends Placeholder {
+  final LibraryElement library;
+  final Identifier node;
+  PrivatePlaceholder(this.library, this.node);
+  String rename(ConflictingRenamer renamer) =>
+      renamer.renamePrivateIdentifier(library, node.source.slowToString());
+  String toString() => 'private_placeholder[node($node), $library]';
+}
+
+class ElementPlaceholder extends Placeholder {
+  final Element element;
+  ElementPlaceholder(this.element);
+  String rename(ConflictingRenamer renamer) => renamer.renameElement(element);
+  String toString() => 'element_placeholder[$element]';
+}