* Ignore dead_code hints for weak-only null checks.

When https://github.com/dart-lang/sdk/issues/41985 is fixed, the
analyzer will begin detecting dead code due to "unnecessary" null
checks.  Since we want to retain null checks in Flutter even when they
appear unnecessary (in order to preserve runtime behavior in weak
mode), we need to suppress these dead code hints.

* Add comments explaining why we're ignoring dead_code hints

This reverts commit 74b5051c.

* Add native stacktrace field for PlatformException

* Mute the readValue check for stacktrace.

* polish

* Add unit test and further polish

* Added more comments

* remove unnecessary import

* fill in stacktrace to JSONMethodCodec and fix tests
Co-authored-by: Ben Li <libe@google.com>

* Updated old button references in dev/integration_tests/android_views

* Empty commit to force all tests to rerun

Bugs:
    https://github.com/flutter/flutter/issues/63241
    https://github.com/flutter/flutter/issues/63242