Convert DartChangeBuilder to use AnalysisSession

pkg/analyzer_plugin/test/support/abstract_context.dart

 // Copyright (c) 2017, 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.
 
 import 'dart:async';
 
+import 'package:analyzer/dart/analysis/session.dart';
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/dart/element/visitor.dart';
 import 'package:analyzer/exception/exception.dart';
 import 'package:analyzer/file_system/file_system.dart';
 import 'package:analyzer/file_system/memory_file_system.dart';
 import 'package:analyzer/source/package_map_resolver.dart';
 import 'package:analyzer/src/dart/analysis/driver.dart';
 import 'package:analyzer/src/dart/analysis/file_state.dart';
 import 'package:analyzer/src/generated/engine.dart';
@@ skipping 37 matching lines @@
   FileContentOverlay _fileContentOverlay = new FileContentOverlay();
   AnalysisDriver _driver;
 
   AnalysisDriver get driver => _driver;
 
   /**
    * Return `true` if strong mode should be enabled for this test.
    */
   bool get enableStrongMode => false;
 
+  /**
+   * Return the analysis session associated with the driver.
+   */
+  AnalysisSession get session => driver.currentSession;
+
   Source addMetaPackageSource() => addPackageSource(
       'meta',
       'meta.dart',
       r'''
 library meta;
 
 const Required required = const Required();
 
 class Required {
   final String reason;
@@ skipping 105 matching lines @@
   final _ElementVisitorFunction function;
 
   _ElementVisitorFunctionWrapper(this.function);
 
   @override
   visitElement(Element element) {
     function(element);
     super.visitElement(element);
   }
 }