This code:
https://github.com/flutter/packages/blob/5bc20b0b42c83bcc01abc98a52a0311e3679de99/packages/pigeon/lib/objc_generator.dart#L968-L969

is incorrect; NSArray and NSDictionary should be copy (since they can be passed mutable versions; they are COW, so it's only less efficient in cases where the current behavior would be wrong anyway).