fix(autocomplete): always set tabindex to allow receiving focus.

Closes #6101 Fixes #5665

src/components/autocomplete/autocomplete.spec.js

@@ -98,6 +98,31 @@ describe('<md-autocomplete>', function() {
       element.remove();
     }));
 
+    it('should allow receiving focus on the autocomplete', function() {
+      var scope = createScope(null, {inputId: 'custom-input-id'});
+      var template = '\
+        <md-autocomplete\
+            md-input-id="{{inputId}}"\
+            md-selected-item="selectedItem"\
+            md-search-text="searchText"\
+            md-items="item in match(searchText)"\
+            md-item-text="item.display"\
+            md-autofocus="true"\
+            placeholder="placeholder">\
+          <span md-highlight-text="searchText">{{item.display}}</span>\
+        </md-autocomplete>';
+      var element = compile(template, scope);
+      var focusSpy = jasmine.createSpy('focus');
+
+      document.body.appendChild(element[0]);
+
+      element.on('focus', focusSpy);
+
+      element.focus();
+
+      expect(focusSpy).toHaveBeenCalled();
+    });
+
     it('should allow you to set an input id without floating label', inject(function() {
       var scope = createScope(null, {inputId: 'custom-input-id'});
       var template = '\

src/components/autocomplete/js/autocompleteDirective.js

@@ -156,9 +156,7 @@ function MdAutocomplete () {
       // Set our variable for the link function above which runs later
       hasNotFoundTemplate = noItemsTemplate ? true : false;
 
-      if (attr.hasOwnProperty('tabindex')) {
-        element.attr('tabindex', '-1');
-      }
+      if (!tabindex) element.attr('tabindex', '-1');
 
       return '\
         <md-autocomplete-wrap\