implement dirty checking for @observable objects

pkg/mdv/lib/src/select_element.dart

Index: pkg/mdv/lib/src/select_element.dart
diff --git a/pkg/mdv/lib/src/select_element.dart b/pkg/mdv/lib/src/select_element.dart
index 34ab99ce11385d64c725956cacea22901e54ca53..4e564c35b59f9ac3ce708fadeecc640a8e5ccb6d 100644
--- a/pkg/mdv/lib/src/select_element.dart
+++ b/pkg/mdv/lib/src/select_element.dart
@@ -51,9 +51,22 @@ class _SelectedIndexBinding extends _InputBinding {
     }
 
     // The binding may wish to bind to an <option> which has not yet been
-    // produced by a child <template>. Delay a maximum of twice -- once for
-    // iterating <optgroup> and once for <option>.
-    var maxRetries = 2;
+    // produced by a child <template>. Furthermore, we may need to wait for
+    // <optgroup> iterating and then for <option>.
+    //
+    // Unlike the JavaScript MDV, we don't have a special "Object.observe" event
+    // loop to schedule on. (See the the "ensureScheduled" function:
+    // https://github.com/Polymer/mdv/commit/9a51ad7ed74a292bf71662cea28acbd151ff65c8)
+    //
+    // Instead we use runAsync. Each <template repeat> needs a delay of 3:
+    //   * once to happen after the child _TemplateIterator is created
+    //   * once to be after _TemplateIterator.inputs CompoundBinding resolve
+    //   * once to be after _TemplateIterator._valueBinding PathObserver fires
+    // And then we need to do this delay sequence twice:
+    //   * once for OPTGROUP
+    //   * once for OPTION.
+    // The resulting 2 * 3 is our maxRetries.
+    var maxRetries = 6;
     delaySetSelectedIndex() {
       if (newValue > element.length && --maxRetries >= 0) {
         runAsync(delaySetSelectedIndex);
@@ -62,11 +75,6 @@ class _SelectedIndexBinding extends _InputBinding {
       }
     }
 
-    // TODO(jmesserly): we aren't matching MDV here. They schedule "model=" to
-    // happen async and then delay using the same scheduler. By using "runAsync"
-    // we're going to happen quite a bit later.
-    // We need to port the "ensureScheduled" function:
-    // https://github.com/Polymer/mdv/commit/9a51ad7ed74a292bf71662cea28acbd151ff65c8
     runAsync(delaySetSelectedIndex);
   }