The spec requires that invalid RegExp literals must cause a Syntax Error
before the JavaScript is executed. See:
https://tc39.es/ecma262/#sec-patterns-static-semantics-early-errors
This is explicitly tested in the RegExp/property-escapes test262 tests.
For example, see unsupported-property-Line_Break.js:
$DONOTEVALUATE();
/\p{Line_Break}/u;
That RegExp literal is invalid because Line_Break is not a supported
Unicode property. $DONOTEVALUATE() just throws an exception when it is
executed. The test expects that this file will fail to be parsed.
Note that RegExp patterns can still be parsed at execution time by way
of "new RegExp(...)".
This allows passing an existing RegExp object (or an object that is
sufficiently like a RegExp object) as the "pattern" argument of the
RegExp constructor.
This is a partial revert of commit 60064e2, which removed the validation
of RegExp flags during runtime and expected the parser to do that
exclusively - however this was not taking into account the RegExp()
constructor, which was subsequently crashing on invalid flags.
Also adds test for these constructor error cases, which were obviously
missing before.
Fixes#7042.
