Return the content of a json file in the library as an object.
@param {String} filePath absolute path to the file
@throws {Error} Throws if file cannot be parsed.
@return {Promise(Object)}
async function(filePath) {
      var data = await fs.readFileAsync(filePath, "utf8");
      try {
        let compileAst = JsonToAst.parseToAst(data, {verbose: true});
        let compileJson = JsonToAst.astToObject(compileAst);
        return compileJson;
      } catch (e) {
        throw new qx.tool.cli.Utils.UserError(`Cannot parse ${filePath}:` + e.message);
      }
    }