Flush InterfaceTypeImpl cached memebers when ClassElement.version is different.

pkg/analyzer/test/src/task/incremental_element_builder_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 analyzer.test.src.task.incremental_element_builder_test;
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/src/dart/ast/utilities.dart';
 import 'package:analyzer/src/dart/element/element.dart';
@@ skipping 1836 matching lines @@
     if (actual != expected) {
       fail('$name\nExpected: $expected\n  Actual: $actual');
     }
   }
 }
 
 class _ClassDeltaHelper {
   final String name;
 
   ClassElementDelta delta;
-  ClassElement element;
+  ClassElementImpl element;
+  int oldVersion;
   List<ClassMember> oldMembers;
   List<ClassMember> newMembers;
 
   _ClassDeltaHelper(this.name);
 
   void initNew(CompilationUnit newUnit, CompilationUnitElementDelta unitDelta) {
+    expect(element.version, isNot(oldVersion));
     ClassDeclaration newClass = _findClassNode(newUnit, name);
     expect(newClass, isNotNull);
     newMembers = newClass.members.toList();
     delta = unitDelta.classDeltas[name];
     expect(delta, isNotNull, reason: 'No delta for class: $name');
   }
 
   void initOld(CompilationUnit oldUnit) {
     ClassDeclaration oldClass = _findClassNode(oldUnit, name);
     expect(oldClass, isNotNull);
     element = oldClass.element;
+    oldVersion = element.version;
     oldMembers = oldClass.members.toList();
   }
 
   ClassDeclaration _findClassNode(CompilationUnit unit, String name) =>
       unit.declarations.singleWhere((unitMember) =>
           unitMember is ClassDeclaration && unitMember.name.name == name);
 }