Increased precision of some flow types

[alesn]

Thanks for taking the effort to create a PR!

If you are creating an extensive PR, you might want to open an issue with your idea first, so that you don't put a lot of effort in an PR that wouldn't be accepted. Please prepend pull requests with WIP: if they are not yet finished
PR checklist:

• Added unit tests
• Updated changelog
• Updated docs (either in the description of this PR as markdown, or as separate PR on the gh-pages branch. Please refer to this PR). For new functionality, at least API.md should be updated
• [ x ] Added flow typings
• Verified that there is no significant performance drop (npm run perf)

Feel free to ask help with any of these boxes!

The above process doesn't apply to doc updates etc.

[coveralls]

Coverage remained the same at 93.717% when pulling b3cf8f4 on alesn:master into 3dba5a6 on mobxjs:master.

[mweststrate]

Thanks!