[dart2js] Clean up asserts for sound nullsafe Dart2js. - Remove asserts that were checking for nulls during migration (via dynamic cast). - Change map key containment asserts from `map[key] == null` to `!map.containsKey(key)` when the map isn't nullable. The null check makes it unclear if the map allows null without looking at the declaration. - Update 2 constructors to use `super.`. Suggested by analyzer when I removed constructor bodies. Change-Id: Idd8969dc84f889adeaaf4024ad6129babc76eb10 Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/338240 Commit-Queue: Nate Biggs <natebiggs@google.com> Reviewed-by: Mayank Patke <fishythefish@google.com>
Dart is:
Optimized for UI: Develop with a programming language specialized around the needs of user interface creation.
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