fix(uiSelect): remove flicker on search change

Removes flicker caused when changing the search string whilst still
preserving the original hiding when dropdown is not visible.

Fixes #1298 and closes #1594
Closes #1557 (Supersedes)

Author: rpocklin

Diff for: src/uiSelectDirective.js

@@ -360,6 +360,8 @@ uis.directive('uiSelect',
           });
         };
 
+        var opened = false;
+        
         scope.calculateDropdownPos = function() {
           if ($select.open) {
             dropdown = angular.element(element).querySelectorAll('.ui-select-dropdown');
@@ -368,8 +370,11 @@ uis.directive('uiSelect',
               return;
             }
 
-            // Hide the dropdown so there is no flicker until $timeout is done executing.
-            dropdown[0].style.opacity = 0;
+           // Hide the dropdown so there is no flicker until $timeout is done executing.
+           if ($select.search === '' && !opened) {
+              dropdown[0].style.opacity = 0;
+              opened = true;
+           }
 
             if (!uisOffset(dropdown).height && $select.$animate && $select.$animate.on && $select.$animate.enabled(dropdown)) {
               var needsCalculated = true;