Build proper reflect polyfill

example.html

@@ -13,7 +13,8 @@
     }
   </style>
   <meta charset="utf-8">
-  <script src="https://cdnjs.cloudflare.com/ajax/libs/webcomponentsjs/1.0.17/webcomponents-lite.js"></script>
+  <script type="text/javascript" src="./reflect.js"></script>
+  <script type="text/javascript" src="https://cdnjs.cloudflare.com/ajax/libs/webcomponentsjs/1.0.20/webcomponents-lite.js"></script>
   <script type="text/javascript" src="dist/index-umd.js"></script>
   <title>image-crop-element demo</title>
 </head>

package-lock.json

@@ -1131,9 +1131,9 @@
       "dev": true
     },
     "core-js": {
-      "version": "1.2.7",
-      "resolved": "https://registry.npmjs.org/core-js/-/core-js-1.2.7.tgz",
-      "integrity": "sha1-ZSKUwUZR2yj6k70tX/KYOk8IxjY=",
+      "version": "2.5.1",
+      "resolved": "https://registry.npmjs.org/core-js/-/core-js-2.5.1.tgz",
+      "integrity": "sha1-rmh03GaTd4m4B1T/VCjfZoGcpQs=",
       "dev": true
     },
     "core-util-is": {
@@ -1648,6 +1648,14 @@
         "promise": "7.3.1",
         "setimmediate": "1.0.5",
         "ua-parser-js": "0.7.17"
+      },
+      "dependencies": {
+        "core-js": {
+          "version": "1.2.7",
+          "resolved": "https://registry.npmjs.org/core-js/-/core-js-1.2.7.tgz",
+          "integrity": "sha1-ZSKUwUZR2yj6k70tX/KYOk8IxjY=",
+          "dev": true
+        }
       }
     },
     "figures": {

package.json

@@ -29,6 +29,7 @@
     "babel-cli": "^6.26.0",
     "babel-plugin-transform-custom-element-classes": "^0.1.0",
     "babel-preset-es2015": "^6.24.1",
+    "core-js": "^2.5.1",
     "eslint": "^4.10.0",
     "eslint-plugin-github": "^0.19.0"
   }