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      <p>Register callbacks to be called when this future completes.</p>
<p>When this future completes with a value,
the <code>onValue</code> callback will be called with that value.
If this future is already completed, the callback will not be called
immediately, but will be scheduled in a later microtask.</p>
<p>If <code>onError</code> is provided, and this future completes with an error,
the <code>onError</code> callback is called with that error and its stack trace.
The <code>onError</code> c