addTempVar(initializer = null) {
    let vars = this.tempVarStack_[this.tempVarStack_.length - 1];
    let name = this.getName_();
    vars.push(new TempVarStatement(name, initializer));
    return name;
  }
Adds a new temporary variable to the current function scope.
@param {ParseTree=} initializer If present then the variable will
    have this as the initializer expression.
@return {string} The name of the temporary variable.