Module not imported for meta-data references.

[mhegazy]

From #12888 (comment)

after upgrading to TypeScript 2.1.5.
the issue regressed in different place.

when 'strictNullCheck' option is false the following typescript emitt invalid javascript code.

file: auxiliry.ts

export class SomeClass {
    field: string;
}

file: main.ts

import { SomeClass } from './auxiliry';

function annotation(): PropertyDecorator {
    return (target: any): void => { };
}

export class ClassA {
    @annotation() array: SomeClass | null;
}

Emitted code:

"use strict";
var __decorate = (this && this.__decorate) || function (decorators, target, key, desc) {
    var c = arguments.length, r = c < 3 ? target : desc === null ? desc = Object.getOwnPropertyDescriptor(target, key) : desc, d;
    if (typeof Reflect === "object" && typeof Reflect.decorate === "function") r = Reflect.decorate(decorators, target, key, desc);
    else for (var i = decorators.length - 1; i >= 0; i--) if (d = decorators[i]) r = (c < 3 ? d(r) : c > 3 ? d(target, key, r) : d(target, key)) || r;
    return c > 3 && r && Object.defineProperty(target, key, r), r;
};
var __metadata = (this && this.__metadata) || function (k, v) {
    if (typeof Reflect === "object" && typeof Reflect.metadata === "function") return Reflect.metadata(k, v);
};
function annotation() {
    return function (target) { };
}
var ClassA = (function () {
    function ClassA() {
    }
    return ClassA;
}());
__decorate([
    annotation(),
    __metadata("design:type", auxiliry_1.SomeClass) // <-- auxiliry_1 is not emitted as require statement
], ClassA.prototype, "array", void 0);
exports.ClassA = ClassA;