Button have a className property (#16098)

According to material-ui documentation, buttons have className property
For example : http://www.material-ui.com/#/components/flat-button

types/material-ui/index.d.ts

@@ -627,6 +627,7 @@ declare namespace __MaterialUI {
     // non generally overridden elements of EnhancedButton
     interface SharedEnhancedButtonProps<T> {
         centerRipple?: boolean;
+        className?: string;
         disableFocusRipple?: boolean;
         disableKeyboardFocus?: boolean;
         disableTouchRipple?: boolean;