[interactive_media_ads] Fixes AdEventTypes not triggering on iOS in release mode

[bparrishMines]

Because _delegate was an unused field, Dart would garbage collect the value in release mode. To prevent this, this adds the @pragma('vm:entry-point') to indicate that the value is an entry point for the platform side. See the comment above the field.

Fixes flutter/flutter#163049

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Footnotes

1. Regular contributors who have demonstrated familiarity with the repository guidelines only need to comment if the PR is not auto-exempted by repo tooling. ↩ ↩² ↩³

[bparrishMines]

Text Exempt reason: Integration tests on CI run in debug mode, but the bug only occurs in release mode.

[stuartmorgan-g]

LGTM

[stuartmorgan-g]

test-exempt: specific to a configuration we can't test in CI

(If we hit more issues like this, maybe a bespoke profile build integration test would work?)

[bparrishMines]

(If we hit more issues like this, maybe a bespoke profile build integration test would work?)

My assumption is that this only happens when the linter finds an unused field because it indicates that a value could be tree-shaken.

Alternatively, I considered adding delegates set with IOSClassName.delegate as associated objects. Especially since this is a unique quirk for the platform. This option is also easier to test.