constructor() {
            this._indexes = { __proto__: null };
            this.array = [];
        }
SetArray acts like a `Set` (allowing only one occurrence of a string `key`), but provides the
index of the `key` in the backing array.

This is designed to allow synchronizing a second array with the contents of the backing array,
like how in a sourcemap `sourcesContent[i]` is the source content associated with `source[i]`,
and there are never duplicates.