Installing the built-ins is not bounded by the heap limit: Math and JSON
no longer reserve heap while they install, so a realm made with a tiny
heap_bytes gets every built-in and runs once the limit is raised. If
installing fails, the realm keeps the reason and every run ends with an
internal error instead of running against missing built-ins.
The json2.js test skips only when the built-in check returns false, and
fails on any other result. The parser's progress line says the app does
not run scripts yet.