Fall back to Javascript

We can't use Dart in Chrome App due to the combination of two reasons:
  * dart2js generates Javascript codes that violates the default CSP
  * Chrome forbids Chrome App to change CSP

So we have to wait either of these happened:
  * Chrome App is allowed to set CSP
  * Dart VM is integrated into Chrome (and thus no dart2js is needed)

Before that, we will be trapped in Javascript :(