c

.babelrc

{
	"presets": ["env"],
	"plugins": [
		"transform-typescript",
		"transform-class-properties"
	]
}

current_output

> npm test

function _classCallCheck(instance, Constructor) { if (!(instance instanceof Constructor)) { throw new TypeError("Cannot call a class as a function"); } }

var A = function A() {
  _classCallCheck(this, A);

  Object.defineProperty(this, "_x", {
    configurable: true,
    enumerable: true,
    writable: true,
    value: void 0
  });
  console.log(2);
};

expected_output

> npm test

function _classCallCheck(instance, Constructor) { if (!(instance instanceof Constructor)) { throw new TypeError("Cannot call a class as a function"); } }

var A = function A() {
  _classCallCheck(this, A);

  console.log(2);
};

index.js

class A {
	private _x: number;

	constructor() {
		console.log(2);
	}
}

package.json

{
  "dependencies": {
    "babel-cli": "^7.0.0-beta.2",
    "babel-plugin-transform-class-properties": "^7.0.0-beta.2",
    "babel-plugin-transform-typescript": "^7.0.0-beta.2",
    "babel-preset-env": "^2.0.0-beta.2"
  },
  "scripts": {
    "test": "babel index.js"
  }
}