Angular does not throw exception for un-assigned bi-directional scope property

[dukehoops-zz]

please see demo - notice that foo is not assigned yet there is no exception in console. Per $compile docs exception should be raised, since foo is non-optional:

If the parent scope property doesn't exist, it will throw a NON_ASSIGNABLE_MODEL_EXPRESSION exception. You can avoid this behavior using =? or =?attr in order to flag the property as optional.

nice to have: IMO attributes should be treated in the same way: required unless ? is used

[panuhorsmalahti]

My patch seems to fix the issue. However, docs/content/error/compile doesn't really have a suitable error message (and it looks like those should be preferred over the documentation's NON_ASSIGNABLE_MODEL_EXPRESSION).

[tbosch]

Note that right now if the value in the isolate scope changes and there is no attribute, there is also a very obscure exception. Solving this issue would also prevent that case early on.

[petebacondarwin]

There is a PR for this #6064