$ cat PureClassExpression.js 
"use strict";

Object.defineProperty(exports, "__esModule", {
  value: true
});

exports["default"] = function () {
  return {
    visitor: {
      ClassExpression: function ClassExpression(path) {
        path.addComment("leading", "#__PURE__");
      }
    }
  };
};

module.exports = exports["default"];

$ echo 'class A{run(){}} class B{jump(){}} f(new A);' | babel --presets env --plugins ./PureClassExpression.js
"use strict";

var _createClass = function () { function defineProperties(target, props) { for (var i = 0; i < props.length; i  ) { var descriptor = props[i]; descriptor.enumerable = descriptor.enumerable || false; descriptor.configurable = true; if ("value" in descriptor) descriptor.writable = true; Object.defineProperty(target, descriptor.key, descriptor); } } return function (Constructor, protoProps, staticProps) { if (protoProps) defineProperties(Constructor.prototype, protoProps); if (staticProps) defineProperties(Constructor, staticProps); return Constructor; }; }();

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

var A = /*#__PURE__*/function () {
  function A() {
    _classCallCheck(this, A);
  }

  _createClass(A, [{
    key: "run",
    value: function run() {}
  }]);

  return A;
}();

var B = /*#__PURE__*/function () {
  function B() {
    _classCallCheck(this, B);
  }

  _createClass(B, [{
    key: "jump",
    value: function jump() {}
  }]);

  return B;
}();

f(new A());

$ uglifyjs -V
uglify-js 2.8.22

$ echo 'class A{run(){}} class B{jump(){}} f(new A);' | babel --presets env --plugins ./PureClassExpression.js | uglifyjs -c toplevel,passes=3 -b
WARN: Dropping __PURE__ call [-:20,21]
WARN: Dropping unused variable B [-:20,4]
"use strict";

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

var _createClass = function() {
    function defineProperties(target, props) {
        for (var i = 0; i < props.length; i  ) {
            var descriptor = props[i];
            descriptor.enumerable = descriptor.enumerable || !1, descriptor.configurable = !0, 
            "value" in descriptor && (descriptor.writable = !0), Object.defineProperty(target, descriptor.key, descriptor);
        }
    }
    return function(Constructor, protoProps, staticProps) {
        return protoProps && defineProperties(Constructor.prototype, protoProps), staticProps && defineProperties(Constructor, staticProps), 
        Constructor;
    };
}(), A = function() {
    function A() {
        _classCallCheck(this, A);
    }
    return _createClass(A, [ {
        key: "run",
        value: function() {}
    } ]), A;
}();

f(new A());

$ echo 'class Bar{} class Foo extends Bar { p = "prop"; static sp1 = 1; static sp2 = 2; baz(){quux()} }' | node_modules/.bin/babel --presets env --plugins babel-plugin-transform-class-properties,./PureClassExpression.js

var Bar = /*#__PURE__*/function Bar() {
  _classCallCheck(this, Bar);
};

var Foo = /*#__PURE__*/function (_Bar) {
  _inherits(Foo, _Bar);

  function Foo() {
    var _ref;

    var _temp, _this, _ret;

    _classCallCheck(this, Foo);

    for (var _len = arguments.length, args = Array(_len), _key = 0; _key < _len; _key  ) {
      args[_key] = arguments[_key];
    }

    return _ret = (_temp = (_this = _possibleConstructorReturn(this, (_ref = Foo.__proto__ || Object.getPrototypeOf(Foo)).call.apply(_ref, [this].concat(args))), _this), _this.p = "prop", _temp), _possibleConstructorReturn(_this, _ret);
  }

  _createClass(Foo, [{
    key: "baz",
    value: function baz() {
      quux();
    }
  }]);

  return Foo;
}(Bar);

Foo.sp1 = 1;    // <-- prevents uglify from dropping unused class IIFE
Foo.sp2 = 2;    // <-- prevents uglify from dropping unused class IIFE

echo 'class Bar{} class Foo extends Bar { p = "prop"; static sp1 = 1; static sp2 = 2; baz(){quux()} }' | node_modules/.bin/babel --presets env --plugins babel-plugin-transform-class-properties,./PureClassExpression.js

var Bar = /*#__PURE__*/function Bar() {
  _classCallCheck(this, Bar);
};

var Foo = /*#__PURE__*/function (_Bar) {
  _inherits(Foo, _Bar);

  function Foo() {
    var _ref;

    var _temp, _this, _ret;

    _classCallCheck(this, Foo);

    for (var _len = arguments.length, args = Array(_len), _key = 0; _key < _len; _key  ) {
      args[_key] = arguments[_key];
    }

    return _ret = (_temp = (_this = _possibleConstructorReturn(this, (_ref = Foo.__proto__ || Object.getPrototypeOf(Foo)).call.apply(_ref, [this].concat(args))), _this), _this.p = "prop", _temp), _possibleConstructorReturn(_this, _ret);
  }

  _createClass(Foo, [{
    key: "baz",
    value: function baz() {
      quux();
    }
  }]);

  Foo.sp1 = 1;    // <-- Note: static properties were moved into the IIFE
  Foo.sp2 = 2;    // <-- Note: static properties were moved into the IIFE
  return Foo;
}(Bar);

$ echo 'class Bar{} class Foo extends Bar { p = "prop"; static sp1 = 1; static sp2 = 2; baz(){quux()} }' | node_modules/.bin/babel --presets env --plugins babel-plugin-transform-class-properties,./PureClassExpression.js | node_modules/.bin/uglifyjs -c toplevel,passes=3
WARN: Side effects in initialization of unused variable _createClass [-:3,4]
WARN: Dropping __PURE__ call [-:15,23]
WARN: Dropping unused variable Foo [-:15,4]
WARN: Dropping unused function defineProperties [-:3,42]
WARN: Dropping side-effect-free statement [-:3,0]
"use strict";