init_() {
    // Populate the prefix tree.
    for (var i = 0; i < this.countries_.length; i++) {
      // Construct key.
      var key = '+' + this.countries_[i].e164_cc;
      // Check if key exists.
      var nodeValue = this.trie_.get(key);
      if (nodeValue) {
        // If so, add country object to its array.
        nodeValue.push(this.countries_[i]);
      } else {
        // Else add that key/value.
        this.trie_.add('+' + this.countries_[i].e164_cc, [this.countries_[i]]);
      }
    }
  }
Populates the prefix tree structure.
@private