Migrate to Rollup + Buble

rollup/demo.js

@@ -0,0 +1,8 @@
+import config from './base-config';
+
+export default Object.assign({}, config, {
+  entry: 'home-assistant-js/demo_data/expose_window.js',
+  targets: [
+    { dest: 'build/_demo_data_compiled.js', format: 'iife' },
+  ],
+});