Update lib/src/plugin/loader.dart

I found an issue by loading text file.
In my case, the issue occurred during the load of /font/fontawesome-webfont.woff?v=3.2.1 from css/font-awesome.css.

    'package:stream/src/plugin/loader.dart': Error: line 92 pos 11: map literal value at index 0 must be a constant of type 'String'
       "json": true, "javascript": true, "dart": true, "xml": true,
              ^