Fix two-way binding in polymer expressions.

pkg/polymer_expressions/test/bindings_test.dart

 // Copyright (c) 2013, 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.
 
 library bindings_test;
 
 import 'dart:async';
 import 'dart:html';
 
 import 'package:observe/observe.dart';
@@ skipping 64 matching lines @@
       });
       return completer.future;
     });
 
     test('should preserve the cursor position', () {
       var model = new NotifyModel('abcde');
       var template = templateBind(new Element.html(
           '<template><input id="i1" value={{x}}></template>'));
       testDiv.append(template.createInstance(model, new PolymerExpressions()));
 
+      var el;
       return new Future(() {
-        var el = testDiv.query("#i1");
+        el = testDiv.query("#i1");
         var subscription = el.onInput.listen(expectAsync((_) {}, count: 1));
         el.focus();
 
         expect(el.value, 'abcde');
         expect(model.x, 'abcde');
 
         el.selectionStart = 3;
         el.selectionEnd = 3;
         expect(el.selectionStart, 3);
         expect(el.selectionEnd, 3);
 
         el.value = 'abc de';
         // Updating the input value programatically (even to the same value in
         // Chrome) loses the selection position.
         expect(el.selectionStart, 6);
         expect(el.selectionEnd, 6);
 
         el.selectionStart = 4;
         el.selectionEnd = 4;
 
         expect(model.x, 'abcde');
         el.dispatchEvent(new Event('input'));
         expect(model.x, 'abc de');
         expect(el.value, 'abc de');
 
         // But propagating observable values through reassign the value and
         // selection will be preserved.
         expect(el.selectionStart, 4);
         expect(el.selectionEnd, 4);
-
         subscription.cancel();
+      }).then(_nextMicrotask).then((_) {

[Jennifer Messerly, 2014/05/07 19:30:46]

fyi -- I wonder if we should use requestAnimationFrame too? that way we know it
is still valid even further in the future--right before a render

[Siggi Cherem (dart-lang), 2014/05/07 19:58:15]

Good idea. PTAL

+        expect(el.selectionStart, 4);
+        expect(el.selectionEnd, 4);
       });
     });
 
 
     test('detects changes to ObservableMap keys/values', () {
       var map = new ObservableMap.from({'a': 1, 'b': 2});
       var template = templateBind(new Element.html('<template>'
           '<template repeat="{{k in x.keys}}">{{k}}:{{x[k]}},</template>'
           '</template>'));
       var model = new NotifyModel(map);
@@ skipping 18 matching lines @@
 @reflectable
 class NotifyModel extends ChangeNotifier {
   var _x;
   NotifyModel([this._x]);
 
   get x => _x;
   set x(value) {
     _x = notifyPropertyChange(#x, _x, value);
   }
 }