Explicitly add lit-element as dependency and update lockfile

package.json

@@ -93,7 +93,8 @@
     "karma-jasmine": "^3.1.1",
     "karma-jasmine-html-reporter": "^1.5.3",
     "karma-webpack": "^4.0.2",
-    "lerna": "^3.20.2",
+    "lerna": "^3.21.0",
+    "lit-element": "^2.3.1",
     "lit-html": "^1.2.1",
     "lodash-template-webpack-loader": "jcbrand/lodash-template-webpack-loader",
     "mini-css-extract-plugin": "^0.9.0",