Add flow-types to library level

[michalkvasnicak]

This PR adds support for flow type without need to configure .flowconfig in a project.

It's basically the same as in apollographql/graphql-tag#97.

This is good for library authors too, for example I wanted to typecheck in my library react-apollo-graphql but I didn't want to force users to configure their .flowconfig just to typecheck apollo-client's stuff. So I made my own typings.

With this PR I can get rid of my own types and use yours :)

Closes #1819

[hunaczech]

+1

[michalkvasnicak]

I tested this in our fairly big project. Fixed a few errors.

For example Request definitions was causing an error because you can't have optional fields and then [extra: string]: any. See try flow.

Also I changed BatchingNetworkInterfaceOptions and NetworkIntefaceOptions definitions to be compatible. So you can assign BatchingNetworkInterfaceOptions to NetworkIntefaceOptions. And changed definition for NetworkInterfaceOptions.uri to string instead of ?string. I thinks this is good, because NetworkInterface is throwing and error if you don't define uri so we can typecheck this so user will get an error sooner than in runtime.

[michalkvasnicak]

Btw index.js.flow could be splitted to multiple files e.g. /transport/networkInterface.js.flow, etc. So keeping it in sync with typescript definitions could be easier and more readable in the future.

But I'm not sure if something like this won't introduce a breaking change because package.main will point to index.js instead of apollo.umd.js.

[stubailo]

I'll leave this to @jbaxleyiii to look at soon!

[jbaxleyiii]

@michalkvasnicak I'll take a look at this today! I agree that breaking it apart would be much better! We talked about doing it the first time around but opted to ship it as an enhancement / make it optional in order to get feedback from users like yourself! Thank you so much for the work on this!

I'll review it and report back today!

[jbaxleyiii]

@michalkvasnicak are you using react-apollo as well? It would be great to do the same setup there if you don't mind!

[jbaxleyiii]

@michalkvasnicak great work! Thank you so much for this!

[jbaxleyiii]

@michalkvasnicak this is great! If you could update the changelog, I'll merge it in and it can be part of the next release!

[michalkvasnicak]

@jbaxleyiii I don't use react-apollo directly, but we use it in another project, I can take a look at flow stuff there as well.

[jbaxleyiii]

@michalkvasnicak thank you again for this! It will be released as 1.8!