Update plugin-proposal-decorators.md

[mountainmoon]

Examples writen in old proposal syntax should be updated since Babel 7.1.0 finally supports the new decorators proposal.

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[nicolo-ribaudo]

We don't support decorator and private elements in the same class yet.

Could you just convert the old examples to the new proposal and add a link to the proposal "for further examples"?

[mountainmoon]

Updated.

[nicolo-ribaudo]

All this function can be simplified to

return function(elementDescriptor) {
  elementDescriptor.descriptor.enumerable = value;
};

[mountainmoon]

I just noticed the examples of the latest proposal were wrote in immutable style. But it looks like not necessary.
So the others can also be simplified to

function annotation(classDescriptor) {
  classDescriptor.elements.push({
    kind: 'field',
    key: 'annotated',
    placement: 'static',
    descriptor: {},
    initializer: () => true,
  });
}

function isTestable(value) {
  return function(classDescriptor) {
    classDescriptor.elements.push({
      kind: 'field',
      key: 'isTestable',
      placement: 'static',
      descriptor: {},
      initializer: () => value,
    });
  };
}

It works in Babel 7.1.0. However, the proposal said a decorator function returns a decorator. So should I add a return ?

return function(elementDescriptor) {
  elementDescriptor.descriptor.enumerable = value;
  return elementDescriptor
};

[nicolo-ribaudo]

If a decorator returns nothing, it returns the original descriptor (probably that introduction needs to be updated, in you want to open a PR).

I'd like to have examples in both styles, to show different ways of using decorators.