const enum imports are stripped even with isolatedModules

[nayeemrmn]

First of all, isolatedModules seems to imply preserveConstEnums but it doesn't look like this is documented: https://www.typescriptlang.org/tsconfig#isolatedModules.

---

Repro:

tsconfig.json:

{
  "compilerOptions": {
    "target": "esnext",
    "isolatedModules": true
  }
}

temp2.ts:

export const enum a {
  b = 1,
  c = 2,
};

temp.ts:

import { a } from "./temp2";
console.log(a.b);

---

temp2.js (enum is generated, so preserveConstEnums is implied?):

export var a;
(function (a) {
    a[a["b"] = 1] = "b";
    a[a["c"] = 2] = "c";
})(a || (a = {}));
;

temp.js (the import is left out, even though no inlining is done??):

console.log(a.b);
export {};

This module is invalid and will throw a reference error at runtime. Expected the import from the TS source to be preserved.

---

I've checked that the same thing will happen with preserveConstEnums specified explicitly. The equivalent issue occurs without "target": "esnext".

Version 3.8.3.

Ref denoland/deno#7326 (comment).

cc @bartlomieju @lucacasonato @kitsonk

[RyanCavanaugh]

I added labels, assignees, and/or a milestone to this issue based on our triage process.

[kitsonk]

In my opinion isolatedModules should throw when it encounters a const enum.