Normalize module identifier lookups with get call

It seems incredibly odd to me that you import using a module name and
then resolve the `get` call with the fully normalized path.  This makes
very little sense to me:

    // Assume System#normalize has been monkey patched.

    System.import('./some/path/to/module').then(function() {
      System.get('/normalized/path/to/module');
    });

This patch ensures that `get` lookups match the import path.