perf(select): Execute render after $digest cycle

This is an optimization to defer execution of the render function in the
select directive after the $digest cycle completes inside the
$watchCollection expressions.  This does a check to see if the render
function is already registered in the $$postDigestQueue before it passes
it into $$postDigest, guaranteeing that the DOM manipulation happens
only in one execution after the model settles.

Author: wesleycho

src/ng/directive/select.js

@@ -413,9 +413,17 @@ var selectDirective = ['$compile', '$parse', function($compile,   $parse) {
 
         ctrl.$render = render;
 
-        scope.$watchCollection(valuesFn, render);
+        scope.$watchCollection(valuesFn, function () {
+          if (scope.$$postDigestQueue.indexOf(render) === -1) {
+            scope.$$postDigest(render);
+          }
+        });
         if ( multiple ) {
-          scope.$watchCollection(function() { return ctrl.$modelValue; }, render);
+          scope.$watchCollection(function() { return ctrl.$modelValue; }, function () {
+            if (scope.$$postDigestQueue.indexOf(render) === -1) {
+              scope.$$postDigest(render);
+            }
+          });
         }
 
         function getSelectedSet() {