Shift from Flow to TypeScript

[tiffon]

In the Flow vs TypeScript comparison, TypeScript has emerged as more robust and popular than Flow.

From the State of JavaScript 2018 Survey:

Language	Would like to learn	Would use again	Total	Source
Flow	24.2%	10.2%	34.6%	link
TypeScript	33.7%	46.7%	80.4%	link

DefinitelyTyped has more type definitions than flow-typed.

There are also many bugs or shortcomings in Flow which do not appear to be issues in TypeScript. For example, in Flow:

class Thing<T, U> {
  static newThing<V>(factory: () => V) {
    const thing: Thing<string, V> = new Thing(factory);
    return thing;
  }

  items: T[];
  factory: () => U;
  constructor(factory: () => U) {
    this.factory = factory;
    this.items = [];
  }
}

const fn: () => number = () => 0;
const t: Thing<string, number> = Thing.newThing<number>(fn);
const t2: Thing<string, number> = Thing.newThing<number>(() => 0);

Results in the following error:

16: const t: Thing<string, number> = Thing.newThing<number>(fn);
                                     ^ Cannot assign `Thing.newThing<...>(...)` to `t` because `V` [1] is incompatible with number [2] in type argument `U` [3].
    References:
    3:     const thing: Thing<string, V> = new Thing(factory);
                                      ^ [1]
    16: const t: Thing<string, number> = Thing.newThing<number>(fn);
                               ^ [2]
    1: class Thing<T, U> {
                      ^ [3]

17: const t2: Thing<string, number> = Thing.newThing<number>(() => 0);
                                      ^ Cannot assign `Thing.newThing<...>(...)` to `t2` because `V` [1] is incompatible with number [2] in type argument `U` [3].
    References:
    3:     const thing: Thing<string, V> = new Thing(factory);
                                      ^ [1]
    17: const t2: Thing<string, number> = Thing.newThing<number>(() => 0);
                                ^ [2]
    1: class Thing<T, U> {
                      ^ [3]

But, this works without issue in TypeScript.

[tiffon]

Fixed by #359.