[web] Unskip assertions test. Update for latest stack trace format.

[ferhatb]

Description

Unskip tests by fixing debugPrintStack for new format.

Related Issues

Fixes #53712

Tests

assertions_test updated.

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[jonahwilliams]

Does this work with stack traces in flutter run? Unfortunately the stack traces are different between the tests which still use build_runner and flutter run which uses the frontend_server.

See also: #52685

[ferhatb]

Does this work with stack traces in flutter run? Unfortunately the stack traces are different between the tests which still use build_runner and flutter run which uses the frontend_server.

See also: #52685

Updated code to handle both build_runner and frontend_server based.

[Piinks]

LGTM! Thanks for the quick fix!