Emit fallback for decorator metadata for type only imports

[rbuckton]

This fixes our emit for decorator metadata when the referenced type comes from a type-only import.

Fixes #37672

[andrewbranch]

How do we know the type is objecty? You can import type a primitive.

[rbuckton]

TypeSerializationKind.ObjectType tells the ts transform to emit Object for the type metadata (which essentially means any). TypeSerializationKind.Unknown means we should try to resolve a value at runtime using that name (by using typeof testing), which would be incorrect behavior. Consider the following using the current version of the compiler (without this change):

// ts
import { dec } from "./decorators";
import type { Map } from "./myMap";

@dec
export class C {
  constructor(map: Map) {}
}

// js
...
let C = /** @class */ (() => {
    var _a;
    let C = class C {
        constructor(map) { }
    };
    C = __decorate([
        dec,
        __metadata("design:paramtypes", [typeof (_a = typeof Map !== "undefined" && Map) === "function" ? _a : Object])
    ], C);
    return C;
})();

The design:paramtypes metadata for C will incorrectly use the global Map object.

For a type-only import we can be certain that the type cannot be referenced as a value, so we should emit Object instead of trying to resolve a value for it.

[liorcode]

Seems this problem still happens in typescript 4.1.3, when using a type which is nested inside a namespace.

For example:

// services.ts

export namespace Services {
  export class Service {}
}


// main.ts
import type { Services } from './services';

declare const decorator: any;
export class Main {
  @decorator()
  fieldTest: Services.Service;
}

JS output:

"use strict";
Object.defineProperty(exports, "__esModule", { value: true });
exports.Main = void 0;
const tslib_1 = require("tslib");
class Main {
}
tslib_1.__decorate([
    decorator(),
    tslib_1.__metadata("design:type", Services.Service)
], Main.prototype, "fieldTest", void 0);
exports.Main = Main;

(Services is undefined)

[andrewbranch]

@liorcode would you mind filing a new issue? Thanks!