Add isWritableFileEntry to the fileSystem API

chrome/test/data/extensions/api_test/file_system/test_util.js

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // This is a duplicate of the file test_util in
 // chrome/test/data/extensions/api_test/file_system
 
 function checkEntry(entry, expectedName, isNew, shouldBeWritable) {
   chrome.test.assertEq(expectedName, entry.name);
+
+  // Test that we are writable (or not), as expected.
+  chrome.fileSystem.isWritableFileEntry(entry, chrome.test.callbackPass(
+      function(isWritable) {
+    chrome.test.assertEq(isWritable, shouldBeWritable);
+  }));
+
   // Test that the file can be read.
   entry.file(chrome.test.callback(function(file) {
     var reader = new FileReader();
     reader.onloadend = chrome.test.callbackPass(function(e) {
       if (isNew)
         chrome.test.assertEq(reader.result, "");
       else
         chrome.test.assertEq(reader.result.indexOf("Can you see me?"), 0);
       // Test that we can write to the file, or not, depending on
       // |shouldBeWritable|.
@@ skipping 33 matching lines @@
         var blob = new Blob(["HoHoHo!"], {type: "text/plain"});
         fileWriter.write(blob);
       });
     });
     reader.onerror = chrome.test.callback(function(e) {
       chrome.test.fail("Error reading file contents.");
     });
     reader.readAsText(file);
   }));
 }