Update for modern browsers

- adopt babel-preset-github (drop IE 11)
- stop transforming custom element classes
- flow strict

Author: mislav

.eslintrc.json

@@ -1,11 +1,30 @@
 {
   "extends": [
-    "plugin:github/es6",
     "plugin:github/browser",
+    "plugin:github/es6",
     "plugin:github/flow"
   ],
   "rules": {
     "github/no-then": "off",
     "no-invalid-this": "off"
-  }
+  },
+  "overrides": [
+    {
+      "files": "test/**/*.js",
+      "rules": {
+        "flowtype/require-valid-file-annotation": "off",
+        "github/unescaped-html-literal": "off"
+      }
+    },
+    {
+      "files": "test/**/*.js",
+      "excludedFiles": "test/karma.config.js",
+      "env": {
+        "mocha": true
+      },
+      "globals": {
+        "assert": true
+      }
+    }
+  ]
 }

README.md

@@ -77,7 +77,6 @@ Browsers without native [custom element support][support] require a [polyfill][]
 - Chrome
 - Firefox
 - Safari
-- Internet Explorer 11
 - Microsoft Edge
 
 [support]: https://caniuse.com/#feat=custom-elementsv1