1. use negative regex (instead of ".") to check iframeError
2. make Test262Error.prototype.toString() always include
   the string Test262Error (no spaces) so it matches /Test262Error/
3. modify check for supportsWindowOnerror - require that
   first argument (message) to onerror() contains the error name
   by checking for /Error:/.

Change (3) above forces IE11 onto the !supportsWindowOnerror path.
Test262 tests are run inside an eval() on IE11, and errors are
caught and explicitly have toString() called.  Without this,
IE11 passes only the `message` property to onerror(), and regexes
that expect to match error name fail.

sth: revert to simple `onerror` checking

gs: refactor function `$DONE`

refactor logic tree
add support for async test failures

ed.js: crude support for error typing

S8.7.1_A2: cache result of delete

`delete` has a side-effect and its
return value depends on this; cache result of
delete so when reporting result, we are always
reporting the result that made us fail, not the
result of a new delete
[pedantic]

ed: explicitly pass cooked error to $DONE

gs: only let $DONE be called once