Version 4.0.0

[danrosenthal]

No description provided.

[BPScott]

Things I'd like to see (a non-exhaustive list):

Code tidying:

• Migrate legacy context API usage to the modern version (this PR starts on this already)
• Remove the dependency on prop-types (it looks like moving to the new context API removes most of our existing needs for this).
• Migrate usage of deprecated lifecycle hooks (this can be done in 3.x releases) - Avoid using deprecated/UNSAFE_* React methods #519
• Strict mode compliance and some way to assert this in tests/storybook (I think the removal of legacy context and deprecated lifecycle hooks are prerequisites for this) - Make Polaris StrictMode compliant #785
• Update minimum version of react/react-dom to at least 16.8 so that we can use hooks.
• Write a withAppProvider hook and use it in place of the withAppProvider() higher-order-component. This ensures means that every component can be a function/class instead of a potentially being wrapped in a HoC. This also means we don't have to do annoying stuff with ComposedProps when building a component. We should probably leave the HoC in place for the release eve if we don't use it internally to give people who consume polaris and may be using it an easier update path

More speculative build stuff:

• Builds solely using Babel for compilation, instead of the TS then Babel two step dance
• Latest typescript version (this can be done in 3.x releases, assuming react-compose context typings breaks Typescript 3.2.x quilt#489 is fixed) (hopefully they'll have merged a fix for Support for --target ES2018 microsoft/TypeScript#20463 by then)

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Whilst "use hooks for ALL THE THINGS" is a laudable goal, and probably where we want to get to eventually, rewriting without adding value probably isn't the best use of time. I think opening the door for using hooks in v4.0.0 is a solid start, and then we can migrate to hooks on a per-component basis in minor releases

[tmlayton]

Would love to see us get over the 90% coverage target

[AndrewMusgrave]

@tmlayton 😎

[alex-page]

I am not going through all of the files, but I have tophatted the changes in styleguide. Looks good to me.

[danrosenthal]

I can't approve my own PR, but consider this my ✅. Nice work, @AndrewMusgrave and @BPScott!