fix MemoryResourceProvider context on Windows

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 18 matching lines @@
       new HashMap<String, List<StreamController<WatchEvent>>>();
   int nextStamp = 0;
 
   final pathos.Context _pathContext;
 
   @override
   final AbsolutePathContext absolutePathContext;
 
   MemoryResourceProvider(
       {pathos.Context context, @deprecated bool isWindows: false})
-      : _pathContext = (context ??= pathos.context),
+      : _pathContext = (context ??= pathos.style == pathos.Style.windows
+            // On Windows, ensure that the current drive matches
+            // the drive inserted by MemoryResourceProvider.convertPath
+            // so that packages are mapped to the correct drive
+            ? new pathos.Context(current: 'C:\\')
+            : pathos.context),
         absolutePathContext =
             new AbsolutePathContext(context.style == 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
@@ skipping 525 matching lines @@
     }
     return path == other.path;
   }
 
   @override
   String toString() => path;
 
   @override
   Uri toUri() => _provider.pathContext.toUri(path);
 }