Fix 'Unused local variable' hints in analyzer and analysis_server.

pkg/analyzer/test/file_system/memory_file_system_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 test.memory_file_system;
 
 import 'dart:async';
 
 import 'package:analyzer/src/generated/engine.dart' show TimestampedData;
 import 'package:analyzer/src/generated/source.dart';
@@ skipping 133 matching lines @@
       });
 
       group('already exists', () {
         test('as folder', () {
           Folder folder = provider.newFolder('/my/folder');
           Folder newFolder = provider.newFolder('/my/folder');
           expect(newFolder, folder);
         });
 
         test('as file', () {
-          File file = provider.newFile('/my/file', 'qwerty');
+          provider.newFile('/my/file', 'qwerty');
           expect(() {
             provider.newFolder('/my/file');
           }, throwsA(new isInstanceOf<ArgumentError>()));
         });
       });
     });
 
     group('modifyFile', () {
       test('nonexistent', () {
         String path = '/my/file';
@@ skipping 338 matching lines @@
         });
 
         test('resolveRelative', () {
           Uri relative = source.resolveRelativeUri(new Uri.file('bar/baz.dart'));
           expect(relative.path, '/foo/bar/baz.dart');
         });
       });
     });
   });
 }