Use Linux error codes as fallback when OS error codes are not defined (#94)

diff --git a/packages/file/lib/src/interface/error_codes.dart b/packages/file/lib/src/interface/error_codes.dart
index 48fc5db..ea278a7 100644
--- a/packages/file/lib/src/interface/error_codes.dart
+++ b/packages/file/lib/src/interface/error_codes.dart
@@ -168,7 +168,7 @@
   static int get EXDEV => _platform((_Codes codes) => codes.exdev);
 
   static int _platform(int getCode(_Codes codes)) {
-    _Codes codes = _platforms[operatingSystem];
+    _Codes codes = _platforms[operatingSystem] ?? _platforms['linux'];
     return getCode(codes);
   }
 }