Adding untested write functionality.

lib/fs.js

@@ -100,3 +100,34 @@ function readFile(path, callback) {
   stream.on("end", callback.bind(null, null, buffer));
 }
 exports.readFile = readFile;
+
+function writeFile(path, content, encoding, callback) {
+  try {
+    if (!callback) {
+      callback = encoding;
+      encoding = undefined;
+    }
+
+    encoding = String(encoding || "utf-8").toUpperCase();
+    let fd = FileDescriptor(path);
+    let outputStream = FileOutputStream();
+    let converter = ConverterStream();
+
+    outputStream.init(fd, 0x02 | 0x08 | 0x20, parseInt("0666"),
+                      outputStream.DEFER_OPEN);
+    converter.charset = encoding;
+    let inputStream = converter.convertToInputStream(content.toString(encoding));
+    let copier = StreamCopier();
+    copier.init(inputStream, outputStream, null, sourceBuffered, sinkBuffered,
+                0x8000, true, true);
+    copier.asyncCopy({
+      onStartRequest: function onStartRequest(aRequest, aContext) {},
+      onStopRequest: function onStopRequest(aRequest, aContext, aStatusCode) {
+        callback(null);
+      }
+    }, null);
+  } catch (error) {
+    callback(error);
+  }
+}
+exports.writeFile = writeFile;