fix(autocomplete): stop loading if last promise got resolved

> Just investigated a bit, the problem is, the autocomplete will set the loading state to false if a previous promise is resolved. So when you search for A-R-I-Z-O-N-A (normally typed - not pasted) there will be a promise for each char. So the state of setLoading will change to false. This can be solved by adding a simple fetching queue. But only check the queue on the promise finally to store the retrieved results in cache.

Fixes #6907

Closes #6927

src/components/autocomplete/js/autocompleteController.js

@@ -19,7 +19,7 @@ function MdAutocompleteCtrl ($scope, $element, $mdUtil, $mdConstant, $mdTheming,
       selectedItemWatchers = [],
       hasFocus             = false,
       lastCount            = 0,
-      promiseFetch         = false;
+      fetchesInProgress    = 0;
 
   //-- public variables with handlers
   defineProperty('hidden', handleHiddenChange, true);
@@ -648,15 +648,16 @@ function MdAutocompleteCtrl ($scope, $element, $mdUtil, $mdConstant, $mdTheming,
       if ( !items ) return;
 
       items = $q.when(items);
+      fetchesInProgress++;
       setLoading(true);
-      promiseFetch = true;
 
       $mdUtil.nextTick(function () {
           items
             .then(handleResults)
             .finally(function(){
-              setLoading(false);
-              promiseFetch = false;
+              if (--fetchesInProgress === 0) {
+                setLoading(false);
+              }
             });
       },true, $scope);
     }
@@ -715,14 +716,18 @@ function MdAutocompleteCtrl ($scope, $element, $mdUtil, $mdConstant, $mdTheming,
     }
   }
 
+  function isPromiseFetching() {
+    return fetchesInProgress !== 0;
+  }
+
   function scrollTo (offset) {
     elements.$.scrollContainer.controller('mdVirtualRepeatContainer').scrollTo(offset);
   }
 
   function notFoundVisible () {
     var textLength = (ctrl.scope.searchText || '').length;
 
-    return ctrl.hasNotFound && !hasMatches() && (!ctrl.loading || promiseFetch) && textLength >= getMinLength() && (hasFocus || noBlur) && !hasSelection();
+    return ctrl.hasNotFound && !hasMatches() && (!ctrl.loading || isPromiseFetching()) && textLength >= getMinLength() && (hasFocus || noBlur) && !hasSelection();
   }
 
   /**