Replace boolean fields in ClassMetadata by an instance kind

[annagrin]

Use InstanceKind from vm service protocol instead of boolean fields in class metadata - it looks like we only need them internally in class.dart.

webdev/dwds/lib/src/debugging/metadata/class.dart

Line 228 in 12f2285

/// TODO(annagrin): convert fields and getters below to kinds.

[annagrin]

Implemented in #2103