Issue 23091. Fix for adding import prefix before string-interpolated identifier.

pkg/analysis_server/test/services/refactoring/rename_import_test.dart

 // Copyright (c) 2014, 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.
 
 library test.services.refactoring.rename_import;
 
 import 'package:analysis_server/src/protocol.dart';
 import 'package:analyzer/src/generated/ast.dart';
 import 'package:unittest/unittest.dart';
 
@@ skipping 48 matching lines @@
 import 'dart:async';
 import 'dart:math' as newName show Random, min hide max;
 main() {
   Future f;
   newName.Random r;
   newName.min(1, 2);
 }
 ''');
   }
 
+  test_createChange_add_interpolationExpression() {
+    indexTestUnit(r'''
+import 'dart:async';
+main() {
+  Future f;
+  print('Future type: $Future');
+}
+''');
+    // configure refactoring
+    _createRefactoring("import 'dart:async");
+    expect(refactoring.refactoringName, 'Rename Import Prefix');
+    refactoring.newName = 'newName';
+    // validate change
+    return assertSuccessfulRefactoring(r'''
+import 'dart:async' as newName;
+main() {
+  newName.Future f;
+  print('Future type: ${newName.Future}');
+}
+''');
+  }
+
   test_createChange_change_className() {
     indexTestUnit('''
 import 'dart:math' as test;
 import 'dart:async' as test;
 main() {
   test.Future f;
 }
 ''');
     // configure refactoring
     _createRefactoring("import 'dart:async");
@@ skipping 98 matching lines @@
     expect(refactoring.refactoringName, 'Rename Import Prefix');
     expect(refactoring.oldName, '');
   }
 
   void _createRefactoring(String search) {
     ImportDirective directive =
         findNodeAtString(search, (node) => node is ImportDirective);
     createRenameRefactoringForElement(directive.element);
   }
 }