function complete() {
    goog.dispose(streamTarget);
    goog.dispose(transport);
    callback.call(opt_scope, allBuffers.length ? allBuffers : null);
  }
Asynchronously snapshots all contexts.
This will take a snapshot of the current context as well as any dependent
ones such as servers or worker threads. The results are sent to the callback
when they have all been returned.
If the call is going to be ignored (no active session) or fails the callback
will fire on the next javascript tick with a null value.

@param {function(this:T, Array.<!wtf.io.Blob>)} callback Function called
    when all buffers are available. The value will be null if an error
    occurred.
@param {T=} opt_scope Callback scope.
@template T