Remove runtime call to assertObject for import statements

Fixes #1115
Yes source -> output.

If the target output code contains destructuring (might be useful for Firefox only apps for example) we do not transform the destructuring here.

But if the input language does not allow destructuring (I'm not sure I can think of a use case for this but it is a possibility with our options) we make sure to transform this here since the destructuring transformer will not do its job later.