Previously, if there was an unhandled exception in an async test, it
might fail to call done() and timeout. Now we have a default "error"
handler to catch unhandled exceptions and fail the test. A few tests
want to actually test the behavior of window.onerror, so they need an
escape hatch.
We now use the "report an exception" AO when a script has an execution
error. This has mostly replaced the older "report the exception" AO in
various specifications. Using this newer AO ensures that
`window.onerror` is invoked when a script has an execution error.