fix embedder on Windows - fixes #25498

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:core' hide Resource;
 
@@ skipping 10 matching lines @@
  */
 class MemoryResourceProvider implements ResourceProvider {
   final Map<String, _MemoryResource> _pathToResource =
       new HashMap<String, _MemoryResource>();
   final Map<String, String> _pathToContent = new HashMap<String, String>();
   final Map<String, int> _pathToTimestamp = new HashMap<String, int>();
   final Map<String, List<StreamController<WatchEvent>>> _pathToWatchers =
       new HashMap<String, List<StreamController<WatchEvent>>>();
   int nextStamp = 0;
 
-  final AbsolutePathContext absolutePathContext =
-      new AbsolutePathContext(false);
+  final Context _pathContext;
+  final AbsolutePathContext absolutePathContext;
+
+  MemoryResourceProvider({bool isWindows: false})
+      : _pathContext = isWindows ? windows : posix,
+        absolutePathContext = new AbsolutePathContext(isWindows);
 
   @override
-  Context get pathContext => posix;
+  Context get pathContext => _pathContext;
 
   /**
    * Delete the file with the given path.
    */
   void deleteFile(String path) {
     _checkFileAtPath(path);
     _pathToResource.remove(path);
     _pathToContent.remove(path);
     _pathToTimestamp.remove(path);
     _notifyWatchers(path, ChangeType.REMOVE);
@@ skipping 74 matching lines @@
     _MemoryFile file = new _MemoryFile(this, path);
     _pathToResource[path] = file;
     _pathToContent[path] = content;
     _pathToTimestamp[path] = stamp != null ? stamp : nextStamp++;
     _notifyWatchers(path, ChangeType.ADD);
     return file;
   }
 
   Folder newFolder(String path) {
     path = pathContext.normalize(path);
-    if (!path.startsWith(pathContext.separator)) {
-      throw new ArgumentError(
-          "Path must start with '${pathContext.separator}' : $path");
+    if (!pathContext.isAbsolute(path)) {
+      throw new ArgumentError("Path must be absolute : $path");
     }
     _MemoryResource resource = _pathToResource[path];
     if (resource == null) {
       String parentPath = pathContext.dirname(path);
       if (parentPath != path) {
         newFolder(parentPath);
       }
       _MemoryFolder folder = new _MemoryFolder(this, path);
       _pathToResource[path] = folder;
       _pathToTimestamp[path] = nextStamp++;
@@ skipping 346 matching lines @@
   bool operator ==(other) {
     if (runtimeType != other.runtimeType) {
       return false;
     }
     return path == other.path;
   }
 
   @override
   String toString() => path;
 }