MatDialogClose shouldn't set aria-label if it has text

[jelbourn]

In cases where a button has both text and a mat-dialog-close, the aria-label should not be set because it trumps the text. We should only set the label if either: the text is empty, or, the only text is within a mat-icon.

It would make sense to add something like Closes the dialog with aria-describedby (via AriaDescriber) if we add an API to customize the message for i18n.

[dgfranklin]

Any status updates on this? @crisbeto , is there anything blocking your pull request?