title: How to make SystemJS transpile 'typescript' based on file extension not contents
body: <p>I have this SystemJS config in index.html:</p>
  
  <pre><code>&lt;body&gt;
          &lt;script src="node_modules/systemjs/dist/system.js"&gt;&lt;/script&gt;
          &lt;script&gt;
              System.config({
                  defaultJSExtensions: true,
                  transpiler: 'typescript',
                  map: {
                      typescript: 'node_modules/typescript/lib/typescript.js'
                  },
                  packages: {
                      "ts": {
                          "defaultExtension": "ts"
                      }
                  },
              });
              System.import('ts/main');
  
          &lt;/script&gt;
  &lt;/body&gt;
  </code></pre>
  
  <p>main.ts:</p>
  
  <pre><code>let a = [1, 2, 3];
  let b = [1, 2, 3];
  </code></pre>
  
  <p>I get: <code>Uncaught SyntaxError: Block-scoped declarations (let, const, function, class) not yet supported outside strict mode</code>. It looks like file is not transpiled by SystemJS.</p>
  
  <p>When I add import statement in first line it works perfectly:</p>
  
  <pre><code>import * as ts from 'typescript'; // or any other package
  
  let a = [1, 2, 3];
  let b = [1, 2, 3];
  </code></pre>
  
  <p>It looks like SystemJS recognizes typescript file by "contents" - is this correct? If yes, how to force it to transpile every <em>.ts or src/</em> file ?</p>