Deprecate the generated AST library and clean up imports

pkg/analysis_server/lib/src/domains/analysis/occurrences_dart.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 domains.analysis.occurrences_dart;
 
 import 'package:analysis_server/plugin/analysis/occurrences/occurrences_core.dart';
 import 'package:analysis_server/src/protocol_server.dart' as protocol;
+import 'package:analyzer/dart/ast/ast.dart';
+import 'package:analyzer/dart/ast/visitor.dart';
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/src/dart/element/element.dart';
 import 'package:analyzer/src/dart/element/member.dart';
-import 'package:analyzer/src/generated/ast.dart';
 import 'package:analyzer/src/generated/engine.dart';
 import 'package:analyzer/src/generated/source.dart';
 
 /**
  * A computer for occurrences in a Dart [CompilationUnit].
  */
 class DartOccurrencesComputer implements OccurrencesContributor {
   @override
   void computeOccurrences(
       OccurrencesCollector collector, AnalysisContext context, Source source) {
@@ skipping 49 matching lines @@
     }
     if (element is PropertyAccessorElement) {
       element = (element as PropertyAccessorElement).variable;
     }
     if (element is Member) {
       element = (element as Member).baseElement;
     }
     return element;
   }
 }