Moved propTypes checking, fixed undefined error (#2464)

Author: krizzu

examples/counter/src/components/Counter.js

@@ -2,19 +2,19 @@ import React, { Component } from 'react'
 import PropTypes from 'prop-types'
 
 class Counter extends Component {
-  static propTypes = {
-    value: PropTypes.number.isRequired,
-    onIncrement: PropTypes.func.isRequired,
-    onDecrement: PropTypes.func.isRequired
+  constructor(props) {
+    super(props);
+    this.incrementAsync = this.incrementAsync.bind(this);
+    this.incrementIfOdd = this.incrementIfOdd.bind(this);
   }
 
-  incrementIfOdd = () => {
+  incrementIfOdd() {
     if (this.props.value % 2 !== 0) {
       this.props.onIncrement()
     }
   }
 
-  incrementAsync = () => {
+  incrementAsync() {
     setTimeout(this.props.onIncrement, 1000)
   }
 
@@ -44,4 +44,10 @@ class Counter extends Component {
   }
 }
 
+Counter.propTypes = {
+  value: PropTypes.number.isRequired,
+  onIncrement: PropTypes.func.isRequired,
+  onDecrement: PropTypes.func.isRequired
+}
+
 export default Counter