Use utf-8 encoding for listing and error page, in http_server VirtualDirectory.

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 98dd963070f488bf8d32ce5d2ed1b8ecc524703b..f34baeaa5a3f5188a870211e02f494a81df342fc 100644
--- a/pkg/http_server/lib/src/virtual_directory.dart
+++ b/pkg/http_server/lib/src/virtual_directory.dart
@@ -254,6 +254,8 @@ class VirtualDirectory {
         return;
       }
 
+      response.headers.contentType =
+          new ContentType('text', 'html', parameters: {'charset': 'utf-8'});
       response.headers.set(HttpHeaders.LAST_MODIFIED, stats.modified);
       var path = Uri.decodeComponent(request.uri.path);
       var encodedPath = new HtmlEscape().convert(path);
@@ -340,6 +342,8 @@ $server
       _errorCallback(request);
       return;
     }
+    response.headers.contentType =
+        new ContentType('text', 'html', parameters: {'charset': 'utf-8'});
     // Default error page.
     var path = Uri.decodeComponent(request.uri.path);
     var encodedPath = new HtmlEscape().convert(path);