Initial Bazel file resolver support.

pkg/analyzer/lib/file_system/file_system.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 analyzer.file_system.file_system;
 
 import 'dart:async';
 
 import 'package:analyzer/src/generated/source.dart';
 import 'package:analyzer/src/util/absolute_path.dart';
@@ skipping 224 matching lines @@
     if (resource is File) {
       return resource.createSource(actualUri ?? uri);
     }
     return null;
   }
 
   @override
   Uri restoreAbsolute(Source source) =>
       _provider.pathContext.toUri(source.fullName);
 
+  ResourceProvider get provider => _provider;
+
   /**
    * Return `true` if the given [uri] is a `file` URI.
    */
   static bool isFileUri(Uri uri) => uri.scheme == FILE_SCHEME;
 }