Use unmock on device in development

[ksaaskil]

• Example of using unmock in running on-device in React Native. In development, where NODE_ENV==="development", switches on Unmock to mock the Cat Facts API.
• Uses unmock from unmock-browser: this seems to be a safer bet than using unmock package, because using that seems to mess things up when running a development server. unmock-browser only overrides global.fetch or window.fetch and cannot have weird sife-effects of messing up other Node.js stuff (might be worth checking if this can be fixed by adding react-native field in the package.json of unmock

Tricks this required:

• global.Buffer needs to be poly-filled with require('buffer').Buffer: [0.59.x] URLSearchParams 'Error: not implemented' facebook/react-native#23922 (comment) Implemented in Polyfill Buffer when missing from global. unmock-js#359
• json-schema-ref-parser depends on http, https and url so metro bundler fails building the project because http, https and url cannot be resolved. The manual fix is to edit node_modules/json-schema-ref-parser/package.json and add http: false and https: false in its browser field (apparently the modules are not needed by unmock-core). The problem with url can be similarly by adding url: false or, preferably, "url": "whatwg-url" Implemented in Use custom json schema faker. unmock-js#360

Future stuff:

• Running tests in the virtual device using Unmock?
• Adding a component for switching Unmock on and off?
• Adding CircleCI integration