Fix the type annotations in the RTDB Query.on/off/once API.#1724

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Feiyang1 merged 1 commit intomasterfrom

revert-1723-revert-1204-mrschmidt-nullability

May 1, 2019

Commits on Apr 24, 2019

Revert "Revert "Fix the type annotations in the RTDB Query.on/off/once API. (#1204)" (#1723)"
schmidt-sebastian
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