reactiveLabelDependency(key, tracker = this._constructorOptions.tracker) {
    if (!key || !tracker) return;

    const genericKey = MongoObject.makeKeyGeneric(key);

    // If in this schema
    if (this._schema[genericKey]) {
      if (!this._depsLabels[genericKey]) {
        this._depsLabels[genericKey] = new tracker.Dependency();
      }
      this._depsLabels[genericKey].depend();
      return;
    }

    // If in subschema
    this.forEachAncestorSimpleSchema(key, (simpleSchema, ancestor, subSchemaKey) => {
      // Pass tracker down so that we get reactivity even if the subschema
      // didn't have tracker option set
      simpleSchema.reactiveLabelDependency(subSchemaKey, tracker);
    });
  }
Add a reactive dependency on the label for a key.
@param key {string}
@param tracker {Tracker}