Make MemoryResourceProvider.convertPath() work with partially posix paths.

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 32 matching lines @@
   @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.style == pathos.windows.style &&
-        path.startsWith(pathos.posix.separator)) {
-      path = r'C:' +
-          path.replaceAll(pathos.posix.separator, pathos.windows.separator);
+    if (pathContext.style == pathos.windows.style) {
+      if (path.startsWith(pathos.posix.separator)) {
+        path = r'C:' + path;
+      }
+      path = 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);
@@ skipping 507 matching lines @@
     }
     return path == other.path;
   }
 
   @override
   String toString() => path;
 
   @override
   Uri toUri() => _provider.pathContext.toUri(path);
 }