Class fields w/esnext+[[Define]]:no shadow error

[sandersn]

With useDefineForClassFields: true and ESNext target, initializer expressions for property declarations are evaluated in the scope of the class body and are permitted to reference parameters or local variables of the constructor. This is different from classic Typescript behaviour, with useDefineForClassFields: false. There, initialisers of property declarations are evaluated in the scope of the constructor body, so constructor-local symbols shadow outer symbols of the same name.

In other words:

var x = 1
class C {
  y = x
  constructor(x: string) { }
}

Is fine with ESNext emit because x refers unambiguously to var x = 1. In non-esnext emit, you'll get

var x = 1
class C {
  constructor(x) {
    y = x // oh no, wrong x!
  }
}

Note that when class fields are accepted in the ECMAScript standard, we'll need to replace ESNext with the current ES20xx.

The emit is already correct for ESNext + useDefineForClassFields, so this PR just removes the error in this case. It leaves the error for other cases, because fixing classic emit is non-trivial and low-value.

Fixes #35085

[elibarzilay]

Two quick comments:

1. It's confusing to figure out why it works to just not throw an error, so maybe a comment would be good there?

2. I think that it's also good to include a test that shows that the local constructor scope is not used when there's no global (which I tried, and it looks like it's working fine...). E.g.,

   class C {
       b = z; // not ok
       constructor(z: string) { }
   }