Add flow check to unit test

[shanshanzhu]

[ ] Add flow check to unit test
This work break the flowtype but pass the unit test (note the build failed b/c of a bazel issue)
#18
https://travis-ci.org/chrisgervang/flowtype-protoc-gen/builds/498487879

[ ] Optional: Add tests that should break flow in unit test.
All the unit test is checking if the types are correct. We can add some unit tests where flow type is incorrect and catch the proper error count.