Fix flow windows

[raphamorim]

Ref: #11703

• update flow to 0.61.0
  • https://github.com/facebook/flow/releases
• getChildHostContext doesn't expect NoContextT, refactoring to accept both.

ReactFiberHostContext.js

For some reason, in Windows scenario it returns NoContextT as Value

const rootInstance = requiredContext(rootInstanceStackCursor.current); // could return NoContextT, not CX
const context = requiredContext(contextStackCursor.current); // could return NoContextT, not C
const nextContext = getChildHostContext(context, fiber.type, rootInstance);

ReactFiberReconciler.js

getChildHostContext(parentHostContext: C, type: T, instance: CX): CX,

some thoughts @gaearon?

flow working: https://ci.appveyor.com/project/raphamorim/react/build/1.0.52

[gaearon]

Thanks. This looks like a Flow bug but let’s work around for now.