Remove forbidden headers

[jonasfj]

@StorminGorman, do you think this would fix issue google/googleapis.dart#73

[jonasfj]

Test passed locally in chrome and dart-vm.

[StorminGorman]

Looks good to me. I haven't found a reason in any dart projects to use a conditional import so that was cool seeing that trick. If any more http/io differences occur I believe it would be best to find a polymorphic solution.

I am curious though and I don't know off hand, would the same errors occur if setting the content-length, and user-agent headers for the simpleUpload() request that occurs during a media upload? That would require a little more looking into and if that is the case then the same forbidden headers should be removed from the code for that as well.

https://github.com/dart-lang/discoveryapis_commons/blob/08d4ef8d78da6f318e8eb0090542681fb8812edd/lib/src/clients.dart#L182

[sigurdm]

I think conditional imports is a too big hammer for this.

You can get the necessary information by:

const forbiddenHeaders = bool.fromEnvironment('dart.library.html') ? <String>['user-agent', 'content-length'] : <String>[];