Change file opening operation to set position at the end of the file when FileMode.APPEND is used.

runtime/bin/file_impl.dart

Index: runtime/bin/file_impl.dart
===================================================================
--- runtime/bin/file_impl.dart	(revision 3943)
+++ runtime/bin/file_impl.dart	(working copy)
@@ -49,8 +49,8 @@
 
 
 class _FileOutputStream implements OutputStream {
-  _FileOutputStream(File file) {
-    _file = file.openSync(FileMode.WRITE);
+  _FileOutputStream(File file, FileMode mode) {
+    _file = file.openSync(mode);
   }
 
   bool write(List<int> buffer, [bool copyBuffer = false]) {
@@ -591,6 +591,9 @@
       }
       if (id != 0) {
         var randomAccessFile = new _RandomAccessFile(id, _name);
+        if (mode == FileMode.APPEND) {
+          _FileUtils.setPosition(id, _FileUtils.length(id));

[Mads Ager (google), 2012/02/07 08:37:31]

Please do this in the C++ code instead. Here you are turning what should be an
async operation into a sync operation for the setting of the position. The C++
code knows about the mode as well and can set the position as part of the open
call. That will happen on a separate thread and will not block the main dart
thread.

[ricow1, 2012/02/07 09:16:22]

Done, I placed these in the platform specific files, let me know if you wan't me
to do something like:
if (file != NULL && mode == kAppend) {
  file->SetPosition(file->Length());
}
in File_Open in file.cc instead (it seems that we normally only do one platform
specific call per file.cc native method so I did the same)  

+        }
         handler(randomAccessFile);
       } else if (_errorHandler != null) {
         _errorHandler("Cannot open file: $_name");
@@ -615,6 +618,9 @@
     if (id == 0) {
       throw new FileIOException("Cannot open file: $_name");
     }
+    if (mode == FileMode.APPEND) {
+      _FileUtils.setPosition(id, _FileUtils.length(id));

[Mads Ager (google), 2012/02/07 08:37:31]

Ditto. This is actually fine because this is the sync version, but if open in
the C++ code takes care of it you don't need this.

[ricow1, 2012/02/07 09:16:22]

Done.

+    }
     return new _RandomAccessFile(id, _name);
   }
 
@@ -647,7 +653,14 @@
 
   InputStream openInputStream() => new _FileInputStream(this);
 
-  OutputStream openOutputStream() => new _FileOutputStream(this);
+  OutputStream openOutputStream([FileMode mode = FileMode.WRITE]) {
+    if (mode != FileMode.WRITE &&
+        mode != FileMode.APPEND) {
+      throw new FileIOException(
+          "Wrong FileMode. Use FileMode.WRITE or FileMode.APPEND");
+    }
+    return new _FileOutputStream(this, mode);
+  }
 
   String get name() => _name;