[Records] dart2js cleanup tasks

[rakudrama]

Cleanup tasks for #49718

• ir/scope_visitor.dart - eager analysis for record initializer
• ir/impact.dart - register record constants
• After https://github.com/dart-lang/language/blob/master/resources/type-system/subtyping.md#rules is updated, update subtyping predicates to match terminology and structure

Also grep for TODO(50081).