finish() {
    const builderInstructions = {};
    for (const zKey in this.buildersByZIndex_) {
      builderInstructions[zKey] = builderInstructions[zKey] || {};
      const builders = this.buildersByZIndex_[zKey];
      for (const builderKey in builders) {
        const builderInstruction = builders[builderKey].finish();
        builderInstructions[zKey][builderKey] = builderInstruction;
      }
    }
    return builderInstructions;
  }
@return {!Object<string, !Object<import("../canvas.js").BuilderType, import("./Builder.js").SerializableInstructions>>} The serializable instructions