Enable loose mode for `class-properties`

Created by: rgrochowicz

Copying the contents of my MobX issue: After trying out the latest create-react-app and mobx 4's decorate, I found that my classes were no longer observable. At first glance, it looks like it's Babel 7's class property transform.

Here's a codesandbox: https://codesandbox.io/s/623qwprm8k I'd expect the updated value of FooBabel7 to trigger the autorun.

Differences in babel output: Source:

class Foo {
  value = 1;
}

Babel 6:

class Foo {
  constructor() {
    this.value = 1;
  }
}

Babel 7:

class Foo {
  constructor() {
    Object.defineProperty(this, "value", {
      configurable: true,
      enumerable: true,
      writable: true,
      value: 1
    });
  }
}

Enabling loose mode reverts the transform back to the Babel 6 behavior.

Simple before/after: Current behavior:

With loose: true:

Original MobX issue: https://github.com/mobxjs/mobx/issues/1471