Method enqueue

src/KeyPromiseQueue.js:10–20

(key, operation)

  }

  enqueue(key, operation) {
    const tuple = this.beforeOp(key);
    const toAwait = tuple[1];
    const nextOperation = toAwait.then(operation);
    const wrappedOperation = nextOperation.then(result => {
      this.afterOp(key);
      return result;
    });
    tuple[1] = wrappedOperation;
    return wrappedOperation;
  }

  beforeOp(key) {
    let tuple = this.queue[key];