[dart2asm] Improve/fix TFA record field dynamic call metadata

[osa1]

In records implementation (https://dart-review.googlesource.com/c/sdk/+/280461) we currently need to consider every record shape class field as dynamically used: (in dispatch_table.dart)

    // Dynamic call metadata is not accurate for record fields, so we consider
    // them to be dynamically called.
    final isRecordMember =
        member.enclosingClass?.superclass == translator.recordInfo.cls;

    final calledDynamically = !isWasmType &&
            (metadata.getterCalledDynamically ||
                metadata.methodOrSetterCalledDynamically ||
                member.name.text == "call") ||
        isRecordMember;

This is because TFA never considers a record class field as dynamically used. For example, for the record (1, 2, a: 'hi') we generate this class:

@pragma('wasm:entry-point')
class Record_2_a {
  @pragma('wasm:entry-point')
  final Object? $1;

  @pragma('wasm:entry-point')
  final Object? $2;

  @pragma('wasm:entry-point')
  final Object? a;

  ...
}

For members $1, $2, and a, TFA generates ProcedureAttributesMetadatas, but getterCalledDynamically of those metadata are always false.

I don't understand TFA in detail so I'm not sure if this is a bug and/or can be improved, but it's worth investigating.

[alexmarkov]

As far as I understand, currently dart2wasm introduces synthetic classes for records, but doesn't lower any of the record types, record field access, record literals and record constants. So, in a kernel program those synthetic classes are just separate entities, unrelated to how record fields are accessed. As a result, we cannot expect from TFA to magically move inferred information about dynamic accesses to record fields into real fields of those synthetic classes.

As we're going along the path of converting records to classes, it would be much cleaner to do the full lowering of records into classes before TFA. That transformation should replace all record types, record field access, record literals and record constants, in addition to just adding record classes. This will ensure that program is consistent and dynamic access to a record field is just a dynamic access to real field of a synthetic class.

[osa1]

As far as I understand, currently dart2wasm introduces synthetic classes for records, but doesn't lower any of the record types, record field access, record literals and record constants.

Correct.

So, in a kernel program those synthetic classes are just separate entities, unrelated to how record fields are accessed. As a result, we cannot expect from TFA to magically move inferred information about dynamic accesses to record fields into real fields of those synthetic classes.

These synthetic classes are added as Record subtypes, so they're not completely invisible in kernel.

As we're going along the path of converting records to classes, it would be much cleaner to do the full lowering of records into classes before TFA. That transformation should replace all record types, record field access, record literals and record constants, in addition to just adding record classes. This will ensure that program is consistent and dynamic access to a record field is just a dynamic access to real field of a synthetic class.

I'm working on this, but the transformation is not entirely trivial, because of loops in DartType nodes. (I don't yet understand why/how/where those loops are formed)

[osa1]

Fix in https://dart-review.googlesource.com/c/sdk/+/286950.