title: Iterating over map with typescript does not work
body: <p>I have a Typescript Map with IDs as keys and booleans as values. I want to iterate over the map and check if at least one is true. I know it would be possible with forEach, but i want to exit the loop after the first was found.</p>
  
  <p>This is my coding (according to <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/for...of#Iterating_over_a_Map" rel="nofollow noreferrer">https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/for...of#Iterating_over_a_Map</a>):</p>
  
  <pre><code>        private isOneSelected(): boolean {
              for (let [key, value] of this.selectionMap) {
                  if (value) {
                     return true;
                  }
              }
              return false;
          }
  </code></pre>
  
  <p>But it is not entering the loop at all.
  The compiled js file looks like this:</p>
  
  <pre><code>            SelectionController.prototype.isOneSelected = function () {
              for (var _i = 0, _a = this.selectionMap; _i &lt; _a.length; _i++) {
                  var _b = _a[_i], key = _b[0], value = _b[1];
                  if (value) {
                      return true;
                  }
              }
              return false;
              };
  </code></pre>
  
  <p>I am compiling against ES6 and i am using the es6-shim typings:
  tsconfig file:</p>
  
  <pre><code>[...]
  "compilerOptions": {
      "sourceMap": true,
      "rootDir": "./client",
      "outDir": ".tmp/client",
      "target": "ES6",
      "module": "commonjs"
  },
  [...]
  </code></pre>
  
  <p>Thanks,</p>
  
  <p>Tobias</p>