update elements and fix some bugs

test/iron_autogrow_textarea_test.dart

 // Copyright (c) 2015, 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.
 @TestOn('browser')
 library polymer_elements.test.iron_autogrow_textarea_test;
 
 import 'dart:async';
 import 'package:polymer_elements/iron_autogrow_textarea.dart';
 import 'package:test/test.dart';
 import 'package:web_components/web_components.dart';
@@ skipping 32 matching lines @@
 
     test('removing rows shrinks the textarea', () {
       IronAutogrowTextarea autogrow = fixture('basic');
       autogrow.bindValue = 'batman\nand\nrobin';
       var initialHeight = autogrow.offsetHeight;
       autogrow.bindValue = 'batman';
       var finalHeight = autogrow.offsetHeight;
       expect(finalHeight < initialHeight, isTrue);
     });
 
+    test('an undefined bindValue is the empty string', () {
+      IronAutogrowTextarea autogrow = fixture('basic');
+      var initialHeight = autogrow.offsetHeight;
+
+      autogrow.bindValue = 'batman\nand\nrobin';
+      var finalHeight = autogrow.offsetHeight;
+      expect(finalHeight , greaterThan(initialHeight));
+
+      autogrow.bindValue = null;
+      expect(autogrow.offsetHeight, initialHeight);
+      expect(autogrow.textarea.value, '');
+    });
+
     test('textarea selection works', () {
       IronAutogrowTextarea autogrow = fixture('basic');
       var textarea = autogrow.textarea;
       autogrow.bindValue = 'batman\nand\nrobin';
 
       autogrow.selectionStart = 3;
       autogrow.selectionEnd = 5;
 
       expect(textarea.selectionStart, 3);
       expect(textarea.selectionEnd, 5);
@@ skipping 35 matching lines @@
       test('a required textarea with no text is invalid', () {
         IronAutogrowTextarea input = fixture('basic');
         input.required = true;
         expect(input.validate(), isFalse);
         input.bindValue = 'batman';
         expect(input.validate(), isTrue);
       });
     }); 
   });
 }