[READ] Step 1: Are you in the right place?

• For issues related to the code in this repository file a Github issue.
• If the issue pertains to Cloud Firestore, read the instructions in the "Firestore issue"
  template.
• For general technical questions, post a question on StackOverflow
  with the firebase tag.
• For general Firebase discussion, use the firebase-talk
  google group.
• For help troubleshooting your application that does not fall under one
  of the above categories, reach out to the personalized
  Firebase support channel.

[REQUIRED] Step 2: Describe your environment

• Operating System version: _____
• Firebase SDK version: 12.1.1
• Firebase Product: auth
• Node.js version: _____
• NPM version: _____

[REQUIRED] Step 3: Describe the problem

Steps to reproduce:

The new error types that are exported in 12.1.1 are great and make error handling much easier. However, the TS interfaces are notably lacking constructors - they just extend all the way to Error. The actual implementations take info: ErrorInfo, message?: string parameters to set the underlying code and message properties.

This means that in testing code I'm not able to really take advantage of the new types without casting or playing other type shenanigans in order to set the properties as expected, which significantly limits the usefulness of these newly exported types.

Relevant Code:

// In a node REPL

> let err = new FirebaseAuthError("test error message")
undefined
> err.code
'auth/undefined'
> err.message
undefined

> let err2 = new FirebaseAuthError({ message: "test error 2", code: "val"  })
undefined
> err2.code
'auth/val'
> err2.message
'test error 2'

// In typescript
// Argument of type '{ message: string; code: string; }' is not assignable to parameter of type 'string'.
const error = new FirebaseAuthError({ message: "test error 2", code: "val"  });

Relates to #2549