fix(button): aria-label injection

* fix to use textContent as aria-label (if not preset)
* fix support for bindings in textContent to update aria-label

Author: ThomasBurleson

src/components/button/button.js

@@ -61,7 +61,7 @@ angular
  *  </md-button>
  * </hljs>
  */
-function MdButtonDirective($mdButtonInkRipple, $mdTheming, $mdAria, $timeout) {
+function MdButtonDirective($mdButtonInkRipple, $mdTheming, $mdAria, $timeout, $interpolate) {
 
   return {
     restrict: 'EA',
@@ -86,14 +86,11 @@ function MdButtonDirective($mdButtonInkRipple, $mdTheming, $mdAria, $timeout) {
   }
 
   function postLink(scope, element, attr) {
-    var node = element[0];
     $mdTheming(element);
     $mdButtonInkRipple.attach(scope, element);
 
-    var elementHasText = node.textContent.trim();
-    if (!elementHasText) {
-      $mdAria.expect(element, 'aria-label');
-    }
+    // Use async expect to support possible bindings in the button label
+    $mdAria.expectWithText(element, 'aria-label');
 
     // For anchor elements, we have to set tabindex manually when the
     // element is disabled

src/components/button/button.spec.js

@@ -1,4 +1,4 @@
-describe('md-button', function() {
+fdescribe('md-button', function() {
 
   beforeEach(module('material.components.button'));
 
@@ -29,6 +29,28 @@ describe('md-button', function() {
     expect($log.warn).not.toHaveBeenCalled();
   }));
 
+  it('should expect an aria-label if element has text content', inject(function($compile, $rootScope, $log) {
+    spyOn($log, 'warn');
+
+    var button = $compile('<md-button>Hello</md-button>')($rootScope);
+    expect(button.attr('aria-label')).toBe("Hello");
+    expect($log.warn).not.toHaveBeenCalled();
+  }));
+
+  it('should set an aria-label if the text content using bindings', inject(function($$rAF, $compile, $rootScope, $log) {
+    spyOn($log, 'warn');
+
+    var scope = angular.extend($rootScope.$new(),{greetings : "Welcome"});
+    var button = $compile('<md-button>{{greetings}}</md-button>')(scope);
+
+    $rootScope.$apply();
+    $$rAF.flush();    // needed for $mdAria.expectAsync()
+
+    expect(button.attr('aria-label')).toBe("Welcome");
+    expect($log.warn).not.toHaveBeenCalled();
+  }));
+
+
   it('should allow attribute directive syntax', inject(function($compile, $rootScope) {
     var button = $compile('<a md-button href="https://google.com">google</a>')($rootScope.$new());
     expect(button.hasClass('md-button')).toBe(true);