Move file_system libraries into 'analyzer'.

pkg/analysis_server/test/abstract_context.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 test.index;
 
-import 'package:analysis_server/src/resource.dart';
+import 'package:analyzer/file_system/file_system.dart';
+import 'package:analyzer/file_system/memory_file_system.dart';
 import 'package:analyzer/src/generated/ast.dart';
 import 'package:analyzer/src/generated/element.dart';
 import 'package:analyzer/src/generated/engine.dart';
 import 'package:analyzer/src/generated/sdk.dart';
 import 'package:analyzer/src/generated/source_io.dart';
 
 import 'mocks.dart';
 import 'reflective_tests.dart';
 
 
@@ skipping 61 matching lines @@
  * [engine.GeneralizingElementVisitor].
  */
 class _ElementVisitorFunctionWrapper extends GeneralizingElementVisitor {
   final _ElementVisitorFunction function;
   _ElementVisitorFunctionWrapper(this.function);
   visitElement(Element element) {
     function(element);
     super.visitElement(element);
   }
 }