Deserialization of timeout in suite metadata fails in dart2wasm

[eyebrowsoffire]

The following line:

test/pkgs/test_api/lib/src/backend/metadata.dart

Line 285 in 9484592

return Timeout(Duration(microseconds: serialized['duration'] as int));

Fails on dart2wasm, because the underlying type is double and cannot be simply cast to an int.