fix embedder on Windows - fixes #25498

pkg/analyzer/lib/file_system/memory_file_system.dart

Index: pkg/analyzer/lib/file_system/memory_file_system.dart
diff --git a/pkg/analyzer/lib/file_system/memory_file_system.dart b/pkg/analyzer/lib/file_system/memory_file_system.dart
index dc676bba812cec7a08264d13cf8fcb10b2c5a085..1066a9d8c2c31b16415e3bba7a3b19e26ce0361d 100644
--- a/pkg/analyzer/lib/file_system/memory_file_system.dart
+++ b/pkg/analyzer/lib/file_system/memory_file_system.dart
@@ -28,11 +28,15 @@ class MemoryResourceProvider implements ResourceProvider {
       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.