remove(key, value) {
    const values = this.map_.get(key);
    if (!values) {
      return false;
    }

    const removed = goog.array.removeIf(values, function(v) {
      'use strict';
      return Object.is(value, v);
    });

    if (removed) {
      this.count_--;
      if (values.length == 0) {
        this.map_.delete(key);
      }
    }
    return removed;
  }
Removes a single occurrence of (key, value) pair.
@param {K} key The key to remove.
@param {V} value The value to remove.
@return {boolean} Whether any matching (key, value) pair is removed.