Fix spark-suggest-box exception when starting to enter text

The exception was: "Uncaught Error: Unsupported operation: Can't use 'label' in reflection because it is not included in a @MirrorsUsed annotation." See discussion of the ways to fix here: https://groups.google.com/a/dartlang.org/forum/#!topic/web/ZcChqcRedgg

TBR: @yjbanov @devoncarew