Error reporting on File in dart:io

runtime/bin/file.dart

Index: runtime/bin/file.dart
diff --git a/runtime/bin/file.dart b/runtime/bin/file.dart
index 9728ab7cd92d338429781fb34974419b3fc74f25..c2eea0d4ec4694b84e13f2b491a3d58024dee770 100644
--- a/runtime/bin/file.dart
+++ b/runtime/bin/file.dart
@@ -365,7 +365,21 @@ interface RandomAccessFile {
 
 
 class FileIOException implements Exception {
-  const FileIOException([String this.message = ""]);
-  String toString() => "FileIOException: $message";
+  const FileIOException([String this.message = "",
+                         OSError this.osError = null]);
+  String toString() {
+    StringBuffer sb = new StringBuffer();
+    sb.add("FileIOException");
+    if (message != null) {
+      sb.add(": $message");
+      if (osError != null) {
+        sb.add(" (${osError.toString()})");
+      }
+    } else if (osError != null) {
+      sb.add(": ${osError.toString()}");
+    }
+    return sb.toString();
+  }
   final String message;
+  final OSError osError;
 }