title: TypeScript VS - compile errors but no design time errors
body: <p>In Visual Studio 2017, I am trying to use ES2015 Promises. Using TypeScript 2.1.5. I have a a tsconfig.json file in the solution, and it looks like this:</p>
  
  <pre><code>{
    "compilerOptions": {
      "noImplicitAny": false,
      "noEmitOnError": true,
      "removeComments": false,
      "sourceMap": true,
      "target": "es5",
      "outFile": "outfile.js",
      "lib": [ "dom",  "es2015.promise", "es5" ]
    },
    "exclude": [
      "node_modules",
      "wwwroot"
    ]
  }
  </code></pre>
  
  <p>I compile, and I get typings errors, for example:</p>
  
  <blockquote>
    <p>error TS2339: Build:Property 'then' does not exist on type
    'Promise'.</p>
  </blockquote>
  
  <p>When I go to the error, I have intellisense showing it does in fact recognize the then function,and I can right click, go to definition, which takes me to lib.es2015.promise.d.ts.</p>
  
  <p>Why does design time work and compile time does not, and how do I fix?</p>