DDC should move away from using JS default arguments for Dart methods with default parameters

[srujzs]

import 'dart:js_interop';

@JS()
external String? get undefined;

void f([String? s = 'default']) {
  print(s);
}

void main() {
  f(undefined);
}

The above prints default on DDC and null on dart2js (also null on dart2wasm, but that's because of how it's converted to null explicitly). This is because DDC lowers f to something like:

function f(s = 'default') {
  dart.print(s);
}

When undefined is passed, JS uses the default value instead. This is generally an okay way to lower this code if undefined never exists in the program, but with JS interop, that can't be guaranteed. Another way to reproduce this discrepancy is converting the above Dart function to JS and then calling it with undefined.

cc @nshahan