[fix] ProgressBar indicator overflow

This allows ProgressBar to correctly render provided styles like
'border-radius' without the indicator bar overflowing.

src/components/ProgressBar/index.js

@@ -11,7 +11,7 @@ import React, { Component } from 'react';
 import { bool, number } from 'prop-types';
 
 class ProgressBar extends Component {
-  _progressElement = null;
+  _progressElement: View;
 
   static displayName = 'ProgressBar';
 
@@ -80,10 +80,12 @@ const styles = StyleSheet.create({
   track: {
     height: 5,
     overflow: 'hidden',
-    userSelect: 'none'
+    userSelect: 'none',
+    zIndex: 0
   },
   progress: {
-    height: '100%'
+    height: '100%',
+    zIndex: -1
   },
   animation: {
     animationDuration: '1s',

src/components/Switch/index.js

@@ -19,7 +19,7 @@ const thumbFocusedBoxShadow = `${thumbDefaultBoxShadow}, 0 0 0 10px rgba(0,0,0,0
 
 class Switch extends PureComponent {
   _checkboxElement: HTMLInputElement;
-  _thumbElement = null;
+  _thumbElement: View;
 
   static displayName = 'Switch';
 

src/components/View/index.js

@@ -70,7 +70,9 @@ class View extends Component {
       otherProps.style.unshift(styles.hasHitSlop);
     }
 
+    // avoid HTML validation errors
     const component = isInAButtonView ? 'span' : 'div';
+
     return createDOMElement(component, otherProps);
   }
 }
@@ -95,7 +97,7 @@ const styles = StyleSheet.create({
   inline: {
     display: 'inline-flex'
   },
-  // this zIndex ordering positions the hitSlop above the View but behind
+  // this zIndex-ordering positions the hitSlop above the View but behind
   // its children
   hasHitSlop: {
     zIndex: 0