getCompletionsAtPosition(fileName, position, preferences, formattingSettings) {
          return this.forwardJSONCall(
            `getCompletionsAtPosition('${fileName}', ${position}, ${preferences}, ${formattingSettings})`,
            () => this.languageService.getCompletionsAtPosition(fileName, position, preferences, formattingSettings)
          );
        }
Get a string based representation of the completions
to provide at the given source position and providing a member completion
list if requested.