Decorators not working for class properties with union type

[vuorinem]

If one of the types in union type is an imported class (say TestClass), it does not get defined for the module. When a decorator is added for the property, TS calls __metadata("design:type", test_class_1.TestClass), but test_class_1 is undefined, causing runtime error.

If the class is used elsewhere within the module, it will get properly defined. So an easy workaround is to call new TestClass() somewhere.

TypeScript Version: 2.2.2

Code

import { TestClass } from "./test-class";

function myDecorator(target, key) {
    console.log("Decorate...");
}

export class Things {
    @myDecorator
    private thing: TestClass | null;
}

Expected behavior:

define(["require", "exports", "./test-class"], function (require, exports, test_class_1) {
    // ...
    __decorate([
        myDecorator,
        __metadata("design:type", test_class_1.TestClass)
    ], Things.prototype, "thing", void 0);
    exports.Things = Things;
});

Actual behavior:

define(["require", "exports"], function (require, exports) {
    // ...
    __decorate([
        myDecorator,
        __metadata("design:type", test_class_1.TestClass)
    ], Things.prototype, "thing", void 0);
    exports.Things = Things;
});

[aluanhaddad]

Reproduced in latest nightly.
It simply blows up on the execution of the class declaration with a ReferenceError exception.

[mhegazy]

Duplicate of #13449