Add find.backButton finder and StandardComponentType enum to find components in tests.

[gspencergoog]

Description

This adds find.backButton() in the Common Finders to allow finding different types of standard UI elements. It works by attaching a key made from an enum value in a new enum called StandardComponentType to all of the standard widgets that perform the associated function.

I also substituted the finder in several places where it is useful in tests.

This allows writing tests that want to find the "back" button without having to know exactly which icon the back button uses under what circumstances. To do it correctly is actually quite complicated, since there are several adaptations that occur (based on platform, and whether it is web or not).

Tests

• Added tests.

[QuncCccccc]

This looks good to me😄! There seems some test failures. Also just thinking whether we should name it more specifically, something like materialIconButton? Because from Material Design, the standard buttons also include other types of button, like ElevatedButton, OutlinedButton etc but MaterialButtonType seems don't have them:)

[gspencergoog]

@goderbauer and I spoke about this, and decided that it's too Material specific, and not extensible enough.

Instead, I'm going to change this to use a well-known set of Keys to search for, with perhaps a convenience function for some of the really common ones (e.g. find.backButton). Then people could even write their own back button implementation, attach the well-known key to it, and be able to have it work in tests as an "official" back button.

[flutter-dashboard]

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[Piinks]

@gspencergoog is this ready for a review? It looks like there are some failing checks.

[gspencergoog]

It's close, I just needed to fix the tests. Let me see what still needs doing and I'll send it off.