NNBD Legacy mode test failures in snapshot_test.cc

[liamappelbe]

Several tests in snapshot_test.cc are failing in NNBD legacy mode (when force enabling non_nullable_flag, but not enabling the experiment flag).

The failures are all due to Dart_CObject that are supposed to have type Dart_CObject_kArray but actually have type Dart_CObject_kUnsupported. Presumably there's some internal type check that's being confused by the legacy nnbd type.

Logs: https://logs.chromium.org/logs/dart/buildbucket/cr-buildbucket.appspot.com/8891173016667186608/+/steps/test_results/0/logs/new_test_failures__logs_/0

[liamappelbe]

This is to do with passing dart objects as serialized messages. The issue is that the native message reader and writer (ApiMessageReader/ApiMessageWriter) only support a small number of types. In particular, for generic types like List, it only supports ints, doubles, and strings as T. So it gets confused when it sees List<int*>, an int with legacy nnbd flag.

Fixing this is a pretty big job involving extending the API, so this is part of #40176.