Method setAsync

src/service/variable-source.js:236–244

* Sets an async value resolver for the variable with the specified name. * The value resolver may optionally take an extra parameter. * Can be called in conjunction with setAsync to allow for additional * asynchronous resolver where expand will use async and expandSync the sync * version

(varName, asyncResolver)

   * @return {!VariableSource}
   */
  setAsync(varName, asyncResolver) {
    devAssert(varName.indexOf('RETURN') == -1);
    this.replacements_[varName] = this.replacements_[varName] || {
      sync: undefined,
      async: undefined,
    };
    this.replacements_[varName].async = asyncResolver;
    return this;
  }

  /**
   * Helper method to set both sync and async resolvers.