__decorate helper should not use `this` when targeting modules

TypeScript Version: 3.7.x-dev.201xxxxx

Search Terms: __decorate this module

Code

class Foo {
  @property() x = 1;
}

Expected behavior:
Top-level this reference is not emitted.

Actual behavior:

The __decrate variable is declared like:

var __decorate = (this && this.__decorate) || function (decorators, target, key, desc) {

Since top-level this is always undefined in modules, (this && this.__decorate) || can be omitted. It's only a few bytes, but the presence of top-level this also causes warnings in other downstream tools like Rollup.

A few other things could be fixed for an ES6+ helper, btw:

• Use const instead of var (allows for some VM optimizations)
• Remove Reflect.decorate
• Don't use arguments

Playground Link: https://www.typescriptlang.org/play/?ts=Nightly#code/MYGwhgzhAEBiD29oG8BQ1oAEAOAne2AprgC4CeAFAJTQAe0AvNAIwDcqAvkA

Related Issues: None?

使用报告的 3.7.x-dev 版本或当前 compiler 重现链接的 TypeScript Playground 示例，并检查生成的 __decorate helper。追踪生成此 helper 的 compiler 入口点；当模块输出不再包含顶层的 this 引用，同时 decorator 行为保持不变时，即表示完成。