Add error handling to directory

runtime/bin/file_impl.dart

Index: runtime/bin/file_impl.dart
diff --git a/runtime/bin/file_impl.dart b/runtime/bin/file_impl.dart
index 45196cf769c2e687d671442fb347b099307beb29..83a4e83e484c6d3dd738e707d18c28be184fa697 100644
--- a/runtime/bin/file_impl.dart
+++ b/runtime/bin/file_impl.dart
@@ -229,6 +229,10 @@ class _FileUtils {
   static final kOSErrorResponse = 2;
   static final kFileClosedResponse = 3;
 
+  static final kErrorResponseErrorType = 0;
+  static final kOSErrorResponseErrorCode = 1;
+  static final kOSErrorResponseMessage = 2;
+
   static List ensureFastAndSerializableBuffer(
       List buffer, int offset, int bytes) {
     // When using the Dart C API to access raw data, using a ByteArray is
@@ -373,13 +377,14 @@ class _FileBase {
   bool _reportError(response, String message) {
     assert(_isErrorResponse(response));
     if (_onError != null) {
-      switch (response[0]) {
+      switch (response[_FileUtil.kErrorResponseErrorType]) {
         case _FileUtils.kIllegalArgumentResponse:
           _onError(new IllegalArgumentException());
           break;
         case _FileUtils.kOSErrorResponse:
-          _onError(new FileIOException(message,
-                                       new OSError(response[2], response[1])));
+          var err = new OSError(response[_FileUtils.kOSErrorResponseErrorCode],
+                                response[_FileUtils.kOSErrorResponseMessage]);
+          _onError(new FileIOException(message, err));
           break;
         case _FileUtils.kFileClosedResponse:
           _onError(new FileIOException("File closed"));