title: Compile problem with LitElement TS example
body: <p>Hello Polymer Community:</p>
  
  <p>New user here having a bit of trouble with lit-element with TS, and not sure why.</p>
  
  <p>I have TS setup and working fine (compiled other TS examples).</p>
  
  <p>Did npm install lit-element per Getting Started page.</p>
  
  <p><a href="https://lit-element.polymer-project.org/guide/start" rel="nofollow noreferrer">https://lit-element.polymer-project.org/guide/start</a></p>
  
  <p>Trying the TS example under the section: "Use LitElement TypeScript decorators" and getting all kinds of bad happening.</p>
  
  <p>tsc my-element.ts results in complete fail:</p>
  
  <pre><code>my-element.ts:14:14 - error TS1219: Experimental support for decorators is a feature that is subject to change in a future release. Set the 'experimentalDecorators' option in your 'tsconfig' or 'jsconfig' to remove this warning.
  
  14 export class MyElement extends LitElement {
  
  my-element.ts:20:3 - error TS1219: Experimental support for decorators is a feature that is subject to change in a future release. Set the 'experimentalDecorators' option in your 'tsconfig' or 'jsconfig' to remove this warning.
  
  20   foo = 'foo';
  
  node_modules/lit-element/lib/updating-element.d.ts:102:38 - error TS2583: Cannot find name 'Map'. Do you need to change your target library? Try changing the `lib` compiler option to es2015 or later.
  
  102 export declare type PropertyValues = Map&lt;PropertyKey, unknown&gt;;
  
  node_modules/lit-html/lib/parts.d.ts:18:63 - error TS2304: Cannot find name 'Iterable'.
  
  18 export declare const isIterable: (value: unknown) =&gt; value is Iterable&lt;unknown&gt;;
  
  node_modules/lit-html/lib/render.d.ts:16:29 - error TS2583: Cannot find name 'WeakMap'. Do you need to change your target library? Try changing the `lib` compiler option to es2015 or later.
  
  16 export declare const parts: WeakMap&lt;Node, NodePart&gt;;
  
  node_modules/lit-html/lib/template-factory.d.ts:56:28 - error TS2583: Cannot find name 'WeakMap'. Do you need to change your target library? Try changing the `lib` compiler option to es2015 or later.
  
  56     readonly stringsArray: WeakMap&lt;TemplateStringsArray, Template&gt;;
  
  node_modules/lit-html/lib/template-factory.d.ts:57:25 - error TS2583: Cannot find name 'Map'. Do you need to change your target library? Try changing the `lib` compiler option to es2015 or later.
  
  57     readonly keyString: Map&lt;string, Template&gt;;
  
  node_modules/lit-html/lib/template-factory.d.ts:59:38 - error TS2583: Cannot find name 'Map'. Do you need to change your target library? Try changing the `lib` compiler option to es2015 or later.
  
  59 export declare const templateCaches: Map&lt;string, templateCache&gt;;
  
  Found 8 errors.
  </code></pre>
  
  <p>I have no idea how to press forward and really want to work with web components and TS for a new project.</p>
  
  <p>My tsconfig.json is:</p>
  
  <pre><code>{
      "compilerOptions": {
        "target": "es6",
        "module": "es6"
      }
  }
  </code></pre>
  
  <p>I have also tried with no tsconfig file. Same results.</p>
  
  <p>Please kind people help point me in the right direction.
  Thank you.
  David</p>