Add dependency on @polymer/decorators for TypeScript plugins

Change-Id: Icf59e9953962d6eb6664efe8ca061268ca4bfd1a
diff --git a/plugins/package.json b/plugins/package.json
index 1d97a64..b5f5d9a 100644
--- a/plugins/package.json
+++ b/plugins/package.json
@@ -3,6 +3,7 @@
     "description": "Gerrit Code Review - Polygerrit plugin dependencies placeholder, expected to be overridden by plugins",
     "browser": true,
     "dependencies": {
+      "@polymer/decorators": "^3.0.0",
       "@polymer/polymer": "^3.4.1",
       "@gerritcodereview/typescript-api": "3.4.2"
     },
diff --git a/plugins/tsconfig.json b/plugins/tsconfig.json
index f9f7712..8598ba1 100644
--- a/plugins/tsconfig.json
+++ b/plugins/tsconfig.json
@@ -3,5 +3,6 @@
     "target": "es2018", /* Specify ECMAScript target version: 'ES3' (default), 'ES5', 'ES2015', 'ES2016', 'ES2017', 'ES2018', 'ES2019' or 'ESNEXT'. */
     "module": "es2015", /* Specify module code generation: 'none', 'commonjs', 'amd', 'system', 'umd', 'es2015', or 'ESNext'. */
     "moduleResolution": "node", /* Specify module resolution strategy: 'node' (Node.js) or 'classic' (TypeScript pre-1.6). */
+    "experimentalDecorators": true
   },
 }
diff --git a/plugins/yarn.lock b/plugins/yarn.lock
index 4d0f91e..4f99eb9 100644
--- a/plugins/yarn.lock
+++ b/plugins/yarn.lock
@@ -7,7 +7,14 @@
   resolved "https://registry.yarnpkg.com/@gerritcodereview/typescript-api/-/typescript-api-3.4.2.tgz#79e8ff336608cbf18e651bfa9541d7bdead5e1f9"
   integrity sha512-iqHd6G43pV4Wk5iNw95AQmWUBuIrY+dvQ1Ne8ZYkOwRhdruh4BAPhMtsmqWDlcVQbfcwZD5F2zFkGB4J4htggw==
 
-"@polymer/polymer@^3.4.1":
+"@polymer/decorators@^3.0.0":
+  version "3.0.0"
+  resolved "https://registry.yarnpkg.com/@polymer/decorators/-/decorators-3.0.0.tgz#e4212ac976d9abd1210f560b6e1be4165c1c0183"
+  integrity sha512-qh+VID9nDV9q3ABvIfWgm7/+udl7v2HKsMLPXFm8tj1fI7qr7yWJMFwS3xWBkMmuNPtmkS8MDP0vqLAQIEOWzg==
+  dependencies:
+    "@polymer/polymer" "^3.0.5"
+
+"@polymer/polymer@^3.0.5", "@polymer/polymer@^3.4.1":
   version "3.4.1"
   resolved "https://registry.yarnpkg.com/@polymer/polymer/-/polymer-3.4.1.tgz#333bef25711f8411bb5624fb3eba8212ef8bee96"
   integrity sha512-KPWnhDZibtqKrUz7enIPOiO4ZQoJNOuLwqrhV2MXzIt3VVnUVJVG5ORz4Z2sgO+UZ+/UZnPD0jqY+jmw/+a9mQ==