Corelib tests have not been migrated to account for web numerics

[Markzipan]

A slew of these have been failing in DDC/Dart2JS for a long time. See: https://dart-current-results.web.app/#/filter=corelib

Example: https://github.com/dart-lang/sdk/blob/main/tests/corelib/int_parse_radix_int64_test.dart

We should guard non-web-numeric-semantics tests using package:expect/variations.dart (such as in https://github.com/dart-lang/sdk/blob/217bfed9bd549fcbb17d6a64ad80d813f4d1236f/tests/corelib/double_floor_test.dart#L6C1-L7C1).