registerTempVarName(name) {
    let vars = this.tempVarStack_[this.tempVarStack_.length - 1];
    vars.push(new TempVarStatement(name, null));
  }
Allows you to create a temp var after you have created a temp identifier.
This is useful if you do not know ahead of time if you need to create the
temp var but you want to pass in a temp name to some function.

Example:
  let name = transformer.getTempIdentifier();
  ...
  if (needed) {
    transformer.registerTempVarName(name);
  } else {
    transformer.releaseTempName(name);
  }

@param {string} name