Move from .sdkext to _sdkext

pkg/analyzer/test/source/sdk_ext_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 test.source.sdk_ext;
 
 import 'package:analyzer/file_system/file_system.dart';
 import 'package:analyzer/file_system/memory_file_system.dart';
 import 'package:analyzer/source/sdk_ext.dart';
 import 'package:unittest/unittest.dart';
 
 MemoryResourceProvider resourceProvider;
 
 buildResourceProvider() {
   resourceProvider = new MemoryResourceProvider();
   resourceProvider.newFolder('/empty');
   resourceProvider.newFolder('/tmp');
-  resourceProvider.newFile('/tmp/.sdkext', r'''
+  resourceProvider.newFile('/tmp/_sdkext', r'''
   {
     "dart:fox": "slippy.dart",
     "dart:bear": "grizzly.dart",
     "dart:relative": "../relative.dart",
     "dart:deep": "deep/directory/file.dart",
     "fart:loudly": "nomatter.dart"
   }''');
 }
 
 clearResourceProvider() {
@@ skipping 59 matching lines @@
       // Restore source's uri.
       var restoreUri = resolver.restoreAbsolute(source);
       expect(restoreUri, isNotNull);
       // Verify that it is 'dart:fox/foo.dart'.
       expect(restoreUri.toString(), equals('dart:fox/foo.dart'));
       expect(restoreUri.scheme, equals('dart'));
       expect(restoreUri.path, equals('fox/foo.dart'));
     });
   });
 }