ui.window.locale should be set on startup

[Hixie]

Currently, it's not, which means we waste 16ms in every app with a MaterialApp because we spin an entire frame waiting for the locale to be set.

This requires some redundant pumps in tests, too.

[HansMuller]

The current version of this problem, once #11497 lands, is that if we have to wait for an app's async localizations to load, then we're going to at least miss the "first" frame. More than one frame if localizations loading actually takes longer than 16ms.

Apps could create initial screens with more than one Localizations widget. That means that we a way for more than one async loader to defer the "first frame".

Maybe WidgetsApp should create a DeferFirstFrame inherited widget. Localizations widgets would use DeferFirstFrame.push() when an async load started and DeferFirstFrame.pop() when the load finished.

[Hixie]

I wouldn't use a widget for this. Just something like binding.lockInput().

[zoechi]

@GaryQian there were some changes that might be related (flutter/engine#6936, #24481).
Do you think this is still an issue?