Only attempt to move dart-sdk folder if the full Dart Editor zip is used...

This fixes #36 and restores functionality when using an SDK zip (as opposed to the Dart Editor + SDK zip).

Wrapped the case where someone might use the huge zip in a conditional. I'm not sure what the use-case is here... really should be only using just the SDK. Heroku also has issues with large repos and this could cause a problem there: https://devcenter.heroku.com/articles/git#repo-size (and Heroku throws warnings when you do this as well).