Test class properties

[Timer]

Before #5182:

 FAIL  fixtures/smoke/issue-5176-flow-class-properties/index.test.js (128.315s)
  ● issue #5176 (flow class properties interaction) › passes tests
    Command failed: ./node_modules/.bin/react-scripts test --env node --ci
    FAIL src/App.test.js
      ✕ creates instance without (7ms)
      ● creates instance without
        TypeError: Cannot read property 'bind' of undefined
          1 | class App {
          2 |   constructor() {
        > 3 |     this.foo = this.foo.bind(this);
            |                         ^
          4 |   }
          5 |   foo: void => void;
          6 |   foo() {

After:

 PASS  fixtures/smoke/issue-5176-flow-class-properties/index.test.js (203.468s)

[gaearon]

Can you write up how to use all this infra somewhere?

[Timer]

Sure. I'll clean it up a bit and toss a guide on usage in CONTRIBUTING.md.