Emitter helper function could be override by user code

Bug Report

🔎 Search Terms

emit, helper

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about _________

⏯ Playground Link

https://www.typescriptlang.org/play?target=1#code/MYewdgzgLgBA+nApgDyosATCMC8MAUAlLgHwwDeMUAFgE4gDuMYiTAorfbUTAL4CwAKCHAANgEMI2AEIUY43DACMfERKkwAIjBRpMMigOGCAbuNowMilk01ER4CCFGIAdKJABzfBlfjCQA

💻 Code

const __extends = () => { throw new Error() }

class B { a = 1 }
class D extends B {}

var d = new D()
console.log(d.a)

🙁 Actual behavior

Exception

🙂 Expected behavior

console.log(1)

Start with the linked TypeScript Playground example and inspect how the compiler emits the __extends helper for the class inheritance code. Reproduce the exception, then trace the emitter's helper handling to determine why the user-defined helper overrides the generated one. Done means the example runs and logs 1 without the user code interfering with the emitted helper.