remove(key, value) {
    if (!this.map_.has(key)) {
      return;
    }
    const newValue = this.map_.get(key).filter((i) => i != value);
    this.map_.set(key, newValue);
    if (!newValue.length) {
      // Delete the array if it's empty, so that |get| will reliably return null
      // "if no such key exists", instead of sometimes returning an empty array.
      this.map_.delete(key);
    }
  }
Remove a specific value, if it exists.
@param {string} key
@param {T} value