fix(dialog): prefix dialog content probably

At the moment, the dialog always retrieved an id of `dialogContent_undefined`

Fixes #7469. Closes #7480

Author: devversion

src/components/dialog/dialog.js

@@ -894,7 +894,7 @@ function MdDialogProvider($$interimElementProvider) {
 
       var role = (options.$type === 'alert') ? 'alertdialog' : 'dialog';
       var dialogContent = element.find('md-dialog-content');
-      var dialogContentId = ('dialogContent_' + element.attr('id')) || ('dialogContent_' + $mdUtil.nextUid());
+      var dialogContentId = 'dialogContent_' + (element.attr('id') || $mdUtil.nextUid());
 
       element.attr({
         'role': role,

src/components/dialog/dialog.spec.js

@@ -181,8 +181,8 @@ describe('$mdDialog', function() {
 
       var dialog = parent.find('md-dialog');
       var content = parent[0].querySelector('md-dialog-content');
-
-      expect(content.id).toMatch(/dialogContent_.*/g);
+      
+      expect(content.id).toMatch(/dialogContent_[0-9]+/g);
     }));
 
     it('should remove `md-dialog-container` on mousedown mouseup and remove', inject(function($mdDialog, $rootScope, $timeout) {