Fix Flow inexact type - bugfix

[klyap]

In some situations, we'd like to pass in an inexact types into the generated functions. However, this throws a Flow error which can be fixed by wrapping types with "readonly": https://github.com/flowtype/flow-bin/issues/125

Without this adjustment, the workarounds are either $FlowFixMe or force casting the input.

[CLAassistant]

All committers have signed the CLA.