Add polyfill and ignore npm debug file

.gitignore

@@ -1,2 +1,3 @@
 node_modules
-build
+build
+npm-debug.log

package.json

@@ -41,6 +41,7 @@
     "test": "jest --watch"
   },
   "dependencies": {
+    "babel-polyfill": "^6.23.0",
     "prop-types": "^15.5.10",
     "react": "^15.6.1",
     "react-dom": "^15.6.1",

webpack.config.js

@@ -5,6 +5,7 @@ module.exports = {
     'webpack-dev-server/client?http://localhost:8080',
     'webpack/hot/only-dev-server',
     'react-hot-loader/patch',
+    'babel-polyfill',
     './src/index.js'
   ],
   module: {

yarn.lock

@@ -783,6 +783,14 @@ babel-plugin-transform-strict-mode@^6.24.1:
     babel-runtime "^6.22.0"
     babel-types "^6.24.1"
 
+babel-polyfill@^6.23.0:
+  version "6.23.0"
+  resolved "https://registry.yarnpkg.com/babel-polyfill/-/babel-polyfill-6.23.0.tgz#8364ca62df8eafb830499f699177466c3b03499d"
+  dependencies:
+    babel-runtime "^6.22.0"
+    core-js "^2.4.0"
+    regenerator-runtime "^0.10.0"
+
 babel-preset-es2015@^6.24.1:
   version "6.24.1"
   resolved "https://registry.yarnpkg.com/babel-preset-es2015/-/babel-preset-es2015-6.24.1.tgz#d44050d6bc2c9feea702aaf38d727a0210538939"