Trailing commas in function parameter and argument lists

Trailing comma in function parameter and argument lists are now allowed.This is an implementation for a Stage-3 ECMAScript proposal that emits down to valid ES3/ES5/ES6.

Example

function foo(
  bar: Bar,
  baz: Baz, // trailing commas are OK in parameter lists
) {
  // Implementation...
}
foo(
  bar,
  baz, // and in argument lists
);