title: tsconfig.json doesn't care about allowJs
body: <p>I am trying to use in my Angular 5 application a .js file which is avaible just in JavaScript language, so I can't find an alternative in TypeScript.</p>
  
  <p>The problem is that I have added the <code>"allowJs": true</code> in my tsconfig.json, but the error is still active. My tsconfig.json is:</p>
  
  <pre><code>{
    "compileOnSave": false,
    "compilerOptions": {
      "outDir": "./dist/out-tsc",
      "allowJs": true,
      "baseUrl": "src",
      "sourceMap": true,
      "declaration": false,
      "moduleResolution": "node",
      "emitDecoratorMetadata": true,
      "experimentalDecorators": true,
      "target": "es5",
      "typeRoots": [
        "node_modules/@types"
      ],
      "lib": [
        "es2016",
        "dom"
      ]
    }
  }
  </code></pre>
  
  <p>The error is: </p>
  
  <pre><code>'&lt;path&gt;/file.js' has unsupported extension. The only supported extensions are '.ts', '.tsx', '.d.ts'.
  </code></pre>
  
  <p>Whats wrong?</p>