A realm made with a low fuel or heap limit lost RegExp and Date, because
installing them compiled a pattern under that limit and the error was
dropped. The built-ins now install before the realm's limits apply.
The early pattern check read an octal class escape starting with 4 to 7
as three digits, while the compiler reads two, so `/[\477-z]/` gave a
SyntaxError for a pattern the compiler accepts. The check now reads
class octal escapes with the compiler's own reader.
docs/target-app-parity.md: the interpreter's entry lists the new
built-ins and the current language-test count.