message: fix feature gate.
diff: @@ -147,10 +147,7 @@ export namespace IModelTile {
  public constructor(majorVersion: number, allowInstancing: boolean) {
  super();
  - let flags = (allowInstancing && IModelApp.tileAdmin.enableInstancing) ? ContentFlags.AllowInstancing : ContentFlags.None;
  - if (IModelApp.tileAdmin.enableImprovedElision)
  - flags = flags | ContentFlags.ImprovedElision;
  -
  + const flags = (allowInstancing && IModelApp.tileAdmin.enableInstancing) ? ContentFlags.AllowInstancing : ContentFlags.None;
  this._prefix = this._separator + majorVersion.toString(16) + this._separator + flags.toString(16) + this._separator;
  }
  @@ -169,7 +166,10 @@ export namespace IModelTile {
  public constructor(allowInstancing: boolean) {
  super();
  - const flags = (allowInstancing && IModelApp.tileAdmin.enableInstancing) ? ContentFlags.AllowInstancing : ContentFlags.None;
  + let flags = (allowInstancing && IModelApp.tileAdmin.enableInstancing) ? ContentFlags.AllowInstancing : ContentFlags.None;
  + if (IModelApp.tileAdmin.enableImprovedElision)
  + flags = flags | ContentFlags.ImprovedElision;
  +
  this._prefix = this._separator + flags.toString(16) + this._separator;
  }
  