Parse loop vars

[epliskin]

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

1. bug fix in ngRepeat and ngOptions directives; 2) new property of $parse service

What is the current behavior? (You can also link to an open issue here)
Non-ascii loop variable names are not valid, even under ngParseExt extension. See issue #15926.

What is the new behavior (if this is a feature change)?
Resolves #15926.

Does this PR introduce a breaking change?
no

Please check if the PR fulfills these requirements

• The commit message follows our guidelines: https://github.com/angular/angular.js/blob/master/CONTRIBUTING.md#commit-message-format
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

Other information:
$parse service is amended with new function property "is_valid_identifier()" which is used by ngOptions and ngRepeat directives to test loop variable names.

[Narretz]

Thanks for the PR - it's good to see that this would be theoretically possible. However, I'm not sure if it makes sense to expose a completely new API for this, as this is developer accessible, and we would need tests for this, too. I also feel like it should be possible to reuse $parse code better.
Side note: the badident error is for ngRepeat, we should not reuse it if we implement this.

I'd love to hear what others have to say, though.

[mgol]

I'd like to avoid exposing the new API as well.