title: PropTypes doesn't work in React
body: <p>I am running React 16.2.0 and I am using PropTypes 15.6.1. I am using ES6 syntax and Webpack.</p>
  
  <p>I am trying to make PropTypes throw a warning when I pass invalid props, but it doesn't work. This is the code:</p>
  
  <p><strong>SimpleMessage.js</strong></p>
  
  <pre><code>import React from "react"
  import PropTypes from "prop-types"
  
  class SimpleMessage extends React.Component {
      render() {
          return(
              &lt;p&gt;{this.props.message}&lt;/p&gt;
          )
      }
  }
  
  SimpleMessage.propTypes = {
      message: PropTypes.func
  }
  
  export default SimpleMessage
  </code></pre>
  
  <p><strong>index.js</strong></p>
  
  <pre><code>import React from "react"
  import ReactDOM from "react-dom"
  
  import SimpleMessage from "./components/SimpleMessage"
  
  window.React = React
  
  ReactDOM.render(
      &lt;SimpleMessage message={"Hello World"} /&gt;,
      document.getElementById("react-container")
  )
  </code></pre>
  
  <p><strong>webpack.config.js</strong></p>
  
  <pre><code>var webpack = require("webpack")
  var path = require("path")
  
  process.noDeprecation = true
  
  module.exports = {
      entry: "./src/index.js",
      output: {
          path: path.join(__dirname, 'dist', 'assets'),
          filename: "bundle.js",
          sourceMapFilename: 'bundle.map'
      },
      devtool: '#source-map',
      module: {
          rules: [
              {
                  test: /\.js$/,
                  exclude: /(node_modules)/,
                  loader: 'babel-loader',
                  query: {
                      presets: ['env', 'stage-0', 'react']
                  }
              },
              {
                  test: /\.css$/,
                  use: ['style-loader','css-loader', {
                      loader: 'postcss-loader',
                      options: {
                        plugins: () =&gt; [require('autoprefixer')]
                      }}]
              },
              {
                  test: /\.scss/,
                  use: ['style-loader','css-loader', {
                      loader: 'postcss-loader',
                      options: {
                        plugins: () =&gt; [require('autoprefixer')]
                      }}, 'sass-loader']
              }
          ]
      },
      plugins: [
          new webpack.DefinePlugin({
              "process.env": {
                  NODE_ENV: JSON.stringify("production")
              }
          }),
          new webpack.optimize.UglifyJsPlugin({
              sourceMap: true,
              warnings: false,
              mangle: false
          })
      ]
  
  }
  </code></pre>
  
  <p>As you might notice I am passing a string ("Hello World") but checking for a function in proptypes. I don't get any errors or warnings by proptypes, and the code runs just fine.</p>