`decorate` incompatible with Babel 7's class properties

[rgrochowicz]

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
    });
  }
}

Versions:
mobx: 4.1.1
babel: 7.0.0-beta.42
react-scripts: 2.0.0-next.b2fd8db8
Chrome: 65.0.3325.181

• Provide a minimal sample reproduction.
• Did you check this issue wasn't filed before?

[mweststrate]

You will have to use this plugin in loose mode. Without that, properties become simply undecoratable until the new decorators proposal is implemented as well. Which is currently WIP and tracked in #1352