dart2dart: Introduce Renamer that is used by renaming unparser

lib/compiler/implementation/tree/renamer.dart

+// 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.
+
+/**
+ * Renames types, identifiers before printing them out.
+ * Does nothing by default.
+ */
+class Renamer {
+  const Renamer();
+
+  /**
+   * Renames type name for given type annotation. Should not touch type
+   * arguments. Returns [null] if no rename is needed.
+   */
+  String renameTypeName(TypeAnnotation type) => null;
+
+  /**
+   * Renames method name. Returns [null] if no rename is needed.
+   */
+  String renameSendMethod(Send send) => null;
+
+  /**
+   * Renames identifier. Returns [null] if no rename is needed.
+   */
+  String renameIdentifier(Identifier node) => null;
+}