[core] Apply 3 different fixes (#5695)

- Prevent using style conflicting properties like border and border Button
facebook/react#6348
- Avoid using ES6 features that can't be transpiled like includes
- Remove dead style properties

src/Stepper/StepLabel.js

@@ -29,8 +29,6 @@ const getStyles = ({active, completed, disabled}, {muiTheme, stepper}) => {
       height: 24,
     },
     iconContainer: {
-      display: 'flex',
-      alignItems: 'center',
       paddingRight: 8,
     },
   };

src/TextField/TextFieldUnderline.js

@@ -75,7 +75,9 @@ const TextFieldUnderline = (props) => {
 
   const styles = {
     root: {
-      border: 'none',
+      borderTop: 'none',
+      borderLeft: 'none',
+      borderRight: 'none',
       borderBottom: 'solid 1px',
       borderColor: borderColor,
       bottom: 8,

src/utils/autoprefixer.js

@@ -26,7 +26,7 @@ export default function(muiTheme) {
       let isFlex = false;
 
       if (isServer) {
-        isFlex = ['flex', 'inline-flex'].includes(style.display);
+        isFlex = ['flex', 'inline-flex'].indexOf(style.display) !== -1;
       }
 
       const stylePrefixed = InlineStylePrefixer.prefixAll(style);