This is just an adaptation of the start project for Polymer 3.0 with support for TypeScript, and it is shamelessly adapted from here. Use this to get a feel for TypeScript Decorators for Polymer, and the basic tsconfig.json.
Please use the latest version of Chrome or Safari to view the app. It uses native dynamic imports. Also, use lit-html plugin for VS Code to get some nice syntax highlighting on template blocks.
npm install -g polymer-cli
git clone https://github.com/twitchax/start-polymer3-typescript.git
cd start-polymer3-typescript
npm install
npm start