This test checks that caching of property accesses doesn't allow for illegal cross-frame access of a custom property. It also checks for an assertion failure that once occurred in this situation.
PASS: customPropertyAccessThrowsException(targetWindow) should be 'true' and is.
PASS: customPropertyAccessThrowsException(targetWindow) should be 'true' and is.
PASS: customPropertyAccessThrowsException(targetWindow) should be 'true' and is.