Fix some failures on the windows bots

pkg/analyzer/lib/file_system/memory_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.memory_file_system;
 
 import 'dart:async';
 import 'dart:collection';
 import 'dart:convert';
 import 'dart:core';
@@ skipping 26 matching lines @@
   MemoryResourceProvider(
       {pathos.Context context, @deprecated bool isWindows: false})
       : _pathContext = context ?? pathos.context,
         absolutePathContext = new AbsolutePathContext(
             pathos.Style.platform == pathos.Style.windows);
 
   @override
   pathos.Context get pathContext => _pathContext;
 
   /**
+   * Convert the given posix [path] to conform to this provider's path context.
+   *
+   * This is a utility method for testing; paths passed in to other methods in
+   * this class are never converted automatically.
+   */
+  String convertPath(String path) {
+    if (pathContext == pathos.windows &&
+        path.startsWith(pathos.posix.separator)) {
+      path = r'C:' +
+          path.replaceAll(pathos.posix.separator, pathos.windows.separator);
+    }
+    return path;
+  }
+
+  /**
    * Delete the file with the given path.
    */
   void deleteFile(String path) {
     _checkFileAtPath(path);
     _pathToResource.remove(path);
     _pathToBytes.remove(path);
     _pathToTimestamp.remove(path);
     _notifyWatchers(path, ChangeType.REMOVE);
   }
 
@@ skipping 492 matching lines @@
   bool operator ==(other) {
     if (runtimeType != other.runtimeType) {
       return false;
     }
     return path == other.path;
   }
 
   @override
   String toString() => path;
 }