title: Cannot get Array.prototype.includes to work in Typescript 2.0
body: <p>I am using typescript@next (version 2.1.0-dev.20160812 to be specific).</p>
  
  <p>I am getting errors when trying to use Arrays.prototype.includes.</p>
  
  <p>For instance this code</p>
  
  <pre><code>let myItems: Array&lt;string&gt;;
  let exists: boolean = myItems.includes('blah');
  </code></pre>
  
  <p>Generates the following error:</p>
  
  <pre><code>Property 'includes' does not exist on type 'string[]'.at line 124 col 26
  </code></pre>
  
  <p>This is my tsconfig.json:</p>
  
  <pre><code>{
    "compileOnSave": true,
    "compilerOptions": {
      "module": "es6",
      "target": "es6",
      "moduleResolution": "node",
      "allowSyntheticDefaultImports": true,
      "noImplicitAny": true,
      "sourceMap": true,
      "outDir": "ts-build",
      "jsx": "preserve"
    },
    "exclude": [
      "node_modules"
    ]
  }
  </code></pre>