Deprecate the generated AST library and clean up imports

pkg/analysis_server/test/services/dependencies/reachable_source_collector_test.dart

 // Copyright (c) 2015, 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.dependencies.import_collector;
 
 import 'package:analysis_server/src/services/dependencies/reachable_source_collector.dart';
-import 'package:analyzer/src/generated/ast.dart';
+import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/src/generated/source.dart';
 import 'package:test_reflective_loader/test_reflective_loader.dart';
 import 'package:unittest/unittest.dart';
 
 import '../../abstract_context.dart';
 import '../../utils.dart';
 
 main() {
   initializeTestEnvironment();
   defineReflectiveTests(ReachableSourceCollectorTest);
@@ skipping 56 matching lines @@
         unorderedEquals([
           'dart:_internal',
           'dart:async',
           'dart:core',
           'dart:math',
           'file:///lib3.dart',
           'file:///lib4.dart'
         ]));
   }
 }