[Select] Fix some W3C issues

Author: oliviertassinari

docs/src/pages/customization/css-in-js/css-in-js.md

@@ -387,6 +387,8 @@ const Styled = createStyled(theme => ({
 }));
 ```
 
+[@jedwards1211](https://github.com/jedwards1211) Has taken the time to move this module into a package: [material-ui-render-props-styles](https://github.com/jcoreio/material-ui-render-props-styles). Feel free to use it.
+
 ### styled-components API (+15 lines)
 
 styled-components's API removes the mapping between components and styles. Using components as a low-level styling construct can be simpler.

packages/material-ui/src/Input/Input.js

@@ -473,7 +473,6 @@ class Input extends React.Component {
         {startAdornment}
         <InputComponent
           aria-invalid={error}
-          aria-required={required}
           autoComplete={autoComplete}
           autoFocus={autoFocus}
           className={inputClassName}

packages/material-ui/src/RadioGroup/RadioGroup.js

@@ -42,13 +42,12 @@ class RadioGroup extends React.Component {
 
     return (
       <FormGroup role="radiogroup" {...other}>
-        {React.Children.map(children, (child, index) => {
+        {React.Children.map(children, child => {
           if (!React.isValidElement(child)) {
             return null;
           }
 
           return React.cloneElement(child, {
-            key: index,
             name,
             inputRef: node => {
               if (node) {

packages/material-ui/src/Select/SelectInput.js

@@ -178,6 +178,7 @@ class SelectInput extends React.Component {
       open: openProp,
       readOnly,
       renderValue,
+      required,
       SelectDisplayProps,
       tabIndex: tabIndexProp,
       type = 'hidden',
@@ -186,6 +187,8 @@ class SelectInput extends React.Component {
     } = this.props;
     const open = this.isOpenControlled && this.displayNode ? openProp : this.state.open;
 
+    delete other['aria-invalid'];
+
     let display;
     let displaySingle = '';
     const displayMultiple = [];
@@ -283,7 +286,6 @@ class SelectInput extends React.Component {
         <input
           value={Array.isArray(value) ? value.join(',') : value}
           name={name}
-          readOnly={readOnly}
           ref={this.handleInputRef}
           type={type}
           {...other}
@@ -411,6 +413,10 @@ SelectInput.propTypes = {
    * @returns {ReactElement}
    */
   renderValue: PropTypes.func,
+  /**
+   * @ignore
+   */
+  required: PropTypes.bool,
   /**
    * Properties applied to the clickable div element.
    */