fix(dialog): add extra classes to identify buttons (#9463)

Adds classes to help identify the confirmation and close buttons in a dialog. This helps with writing tests that involve the dialog, because it doesn't depend on DOM order.

Author: crisbeto

src/components/dialog/dialog.js

@@ -600,10 +600,10 @@ function MdDialogProvider($$interimElementProvider) {
         '  </md-dialog-content>',
         '  <md-dialog-actions>',
         '    <md-button ng-if="dialog.$type === \'confirm\' || dialog.$type === \'prompt\'"' +
-        '               ng-click="dialog.abort()" class="md-primary">',
+        '               ng-click="dialog.abort()" class="md-primary md-cancel-button">',
         '      {{ dialog.cancel }}',
         '    </md-button>',
-        '    <md-button ng-click="dialog.hide()" class="md-primary" md-autofocus="dialog.$type===\'alert\'">',
+        '    <md-button ng-click="dialog.hide()" class="md-primary md-confirm-button" md-autofocus="dialog.$type===\'alert\'">',
         '      {{ dialog.ok }}',
         '    </md-button>',
         '  </md-dialog-actions>',