Function appendIfUnique

test/fixtures/snapshot/typescript.js:1226–1234

* Unlike `pushIfUnique`, this can take `undefined` as an input, and returns a new array.

(array, toAdd, equalityComparer)

     * Unlike `pushIfUnique`, this can take `undefined` as an input, and returns a new array.
     */
    function appendIfUnique(array, toAdd, equalityComparer) {
        if (array) {
            pushIfUnique(array, toAdd, equalityComparer);
            return array;
        }
        else {
            return [toAdd];
        }
    }
    ts.appendIfUnique = appendIfUnique;
    function stableSortIndices(array, indices, comparer) {
        // sort indices by value then position