message: Fix the evaluation of "label" attributes of text techniques.
  The current code was assuming that label attributes of text
  techniques were always string literals. Of course, this
  is not the case when using expressions.
diff: @@ -491,7 +491,11 @@ export namespace ExtendedTileInfo {
  // tslint:disable-next-line: deprecation
  if (technique.label !== undefined) {
  // tslint:disable-next-line: deprecation
  - const name = env.lookup(technique.label);
  + const attributeName = evaluateTechniqueAttr(context, technique.label);
  + if (typeof attributeName !== "string") {
  + return undefined;
  + }
  + const name = env.lookup(attributeName);
  return typeof name === "string" ? name : undefined;
  }
  // tslint:disable-next-line: deprecation
  