Fix erroneous restoration of task model state.

pkg/analyzer/test/generated/incremental_resolver_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 engine.incremental_resolver_test;
 
 import 'package:analyzer/src/context/cache.dart' as task;
 import 'package:analyzer/src/context/cache.dart';
 import 'package:analyzer/src/generated/ast.dart';
 import 'package:analyzer/src/generated/element.dart';
@@ skipping 3503 matching lines @@
     _updateAndValidate(r'''
 class A {
   void foo() {}
   void foo2() {}
 }
 ''');
   }
 
   @override
   void setUp() {
-    AnalysisEngine.instance.useTaskModel = true;
     super.setUp();
     _resetWithIncremental(true);
   }
 
-  @override
-  void tearDown() {
-    super.tearDown();
-    AnalysisEngine.instance.useTaskModel = false;
-  }
-
   void test_computeConstants() {
     _resolveUnit(r'''
 int f() => 0;
 main() {
   const x = f();
   print(x + 1);
 }
 ''');
     _updateAndValidate(r'''
 int f() => 0;
@@ skipping 1383 matching lines @@
     return ResolutionContextBuilder.contextFor(node, listener).scope;
   }
 }
 
 class _Edit {
   final int offset;
   final int length;
   final String replacement;
   _Edit(this.offset, this.length, this.replacement);
 }