though not recommended, the returned instance may have its <code>error</code> member
(<a href="dart-async/AsyncError/error.html">AsyncError.error</a>) be equal to <code>null</code> in which case the error should be
replaced by a <a href="dart-core/NullThrownError-class.html">NullThrownError</a>.</p>
<p>Custom zones may intercept this operation.</p>
<p>Implementations of a new asynchronous primitive that converts synchronous
errors to asynchronous errors rarely need to invoke <a href="dart-async/Zone/errorCa