Method withContext

packages/compiler/src/expression_parser/parser.ts:693–698

* Executes a callback in the provided context.

(context: ParseContextFlags, cb: () => T)

   * Executes a callback in the provided context.
   */
  private withContext<T>(context: ParseContextFlags, cb: () => T): T {
    this.context |= context;
    const ret = cb();
    this.context ^= context;
    return ret;
  }

  private consumeOptionalCharacter(code: number): boolean {
    if (this.next.isCharacter(code)) {