Fix File.writeAsString to use named optional arguments.

tests/standalone/io/file_write_as_test.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import 'dart:io';
 import 'dart:isolate';
 
 testWriteAsBytesSync(dir) {
   var f = new File('${dir.path}/bytes_sync.txt');
   var data = [50,50,50];
   f.writeAsBytesSync(data);
   Expect.listEquals(data, f.readAsBytesSync());
   f.writeAsBytesSync(data, FileMode.APPEND);
   var expected = [50, 50, 50, 50, 50, 50];
   Expect.listEquals(expected, f.readAsBytesSync());
 }
 
 testWriteAsStringSync(dir) {
   var f = new File('${dir.path}/string_sync.txt');
   var data = 'asdf';
   f.writeAsStringSync(data);
   Expect.equals(data, f.readAsStringSync());
-  f.writeAsStringSync(data, FileMode.APPEND);
+  f.writeAsStringSync(data, mode: FileMode.APPEND);
   Expect.equals('$data$data', f.readAsStringSync());
 }
 
 Future testWriteAsBytes(dir) {
   var completer = new Completer();
   var f = new File('${dir.path}/bytes.txt');
   var data = [50,50,50];
   f.writeAsBytes(data).then((file){
     Expect.equals(f, file);
     f.readAsBytes().then((bytes) {
@@ skipping 12 matching lines @@
 }
 
 Future testWriteAsString(dir) {
   var completer = new Completer();
   var f = new File('${dir.path}/strings.txt');
   var data = 'asdf';
   f.writeAsString(data).then((file){
     Expect.equals(f, file);
     f.readAsString().then((str) {
       Expect.equals(data, str);
-      f.writeAsString(data, FileMode.APPEND).then((file) {
+      f.writeAsString(data, mode: FileMode.APPEND).then((file) {
         Expect.equals(f, file);
         f.readAsString().then((str) {
           Expect.equals('$data$data', str);
           completer.complete(true);
         });
       });
     });
   });
   return completer.future;
 }
 
 main() {
   var port = new ReceivePort();
   var tempDir = new Directory('').createTempSync();
   testWriteAsBytesSync(tempDir);
   testWriteAsStringSync(tempDir);
   testWriteAsBytes(tempDir).chain((_) {
     return testWriteAsString(tempDir);
   }).then((_) {
     tempDir.deleteSync(recursive: true);
     port.close();
   });
 }