Use toNativePath() in http_server pkg, to get the native path string on Windows.

pkg/http_server/lib/src/virtual_directory.dart

Index: pkg/http_server/lib/src/virtual_directory.dart
diff --git a/pkg/http_server/lib/src/virtual_directory.dart b/pkg/http_server/lib/src/virtual_directory.dart
index a09613527008cdebc12d9ea51a3f1192fe9c0cbd..680781e1c501eef310b210944639686fd49e596f 100644
--- a/pkg/http_server/lib/src/virtual_directory.dart
+++ b/pkg/http_server/lib/src/virtual_directory.dart
@@ -87,7 +87,7 @@ class _VirtualDirectory implements VirtualDirectory {
 
   Future<FileSystemEntity> _locateResource(Path path,
                                            Iterable<String> segments) {
-    return FileSystemEntity.type(path.toString(), followLinks: false)
+    return FileSystemEntity.type(path.toNativePath(), followLinks: false)
         .then((type) {
           switch (type) {
             case FileSystemEntityType.FILE:
@@ -108,7 +108,6 @@ class _VirtualDirectory implements VirtualDirectory {
                 // TODO
               }
               break;
-
           }
           // Return `null` on fall-through, to indicate NOT_FOUND.
           return null;