Add Rollup and a checksize npm script

rollup.config.js

@@ -0,0 +1,47 @@
+/**
+ * @license
+ * Copyright (c) 2018 The Polymer Project Authors. All rights reserved.
+ * This code may only be used under the BSD style license found at
+ * http://polymer.github.io/LICENSE.txt
+ * The complete set of authors may be found at
+ * http://polymer.github.io/AUTHORS.txt
+ * The complete set of contributors may be found at
+ * http://polymer.github.io/CONTRIBUTORS.txt
+ * Code distributed by Google as part of the polymer project is also
+ * subject to an additional IP rights grant found at
+ * http://polymer.github.io/PATENTS.txt
+ */
+
+import filesize from 'rollup-plugin-filesize';
+import {terser} from 'rollup-plugin-terser';
+import resolve from 'rollup-plugin-node-resolve';
+import replace from '@rollup/plugin-replace';
+
+export default {
+  input: 'my-element.js',
+  output: {
+    file: 'my-element.bundled.js',
+    format: 'esm',
+  },
+  onwarn(warning) {
+    if (warning.code !== 'THIS_IS_UNDEFINED') {
+      console.error(`(!) ${warning.message}`);
+    }
+  },
+  plugins: [
+    replace({'Reflect.decorate': 'undefined'}),
+    resolve(),
+    terser({
+      module: true,
+      warnings: true,
+      mangle: {
+        properties: {
+          regex: /^__/,
+        },
+      },
+    }),
+    filesize({
+      showBrotliSize: true,
+    })
+  ]
+}