AMD __transpiledModule flag for System loader transpilation support

To explain the feature, the reason for this is I want to be able to dynamically load AMD from ES6 Module Loader, but also support loading AMD that is compiled from ES6 and should be treated like an ES6 module.

In this way, one can write ES6 modules, then transpile them back into AMD for production, and have the `System` loader behave identically, just without the Traceur step.

But, in order to distinguish handling between an AMD module and an ES6 in disguise as an AMD module, we need to have some kind of flag to know that we should treat it as the module object directly:

```javascript
  new Module(amdExport)
```

instead of using the `default` property of a module, which is done for AMD exports normally for compatibility:

```javascript
  new Module({ default: amdExport })
```

Suggestions welcome if there are better ways of going about this.

Review URL: https://github.com/google/traceur-compiler/pull/683

Closes #683.