  While there is mounting evidence that long Gamma-Ray Bursts (GRBs) are
associated with the collapse of massive stars, the detailed structure of their
pre-supernova stage is still debatable. Particularly uncertain is the degree of
mixing among shells of different composition, and hence the role of magnetic
torques and convection in transporting angular momentum. Here we show that
early-time afterglow observations with the Swift satellite place constraints on
the allowed GRB pre-supernova models. In particular, they argue against
pre-supernova models in which different elemental shells are unmixed. These
types of models would produce energy injections into the GRB engine on
timescales between several hundreds of seconds to a few hours. Flaring activity
has {\em not} been observed in a large fraction of well-monitored long GRBs.
Therefore, if the progenitors of long GRBs have common properties, then the
lack of flares indicates that the massive stars which produce GRBs are mostly
well mixed, as expected in low-metallicity, rapidly rotating massive stars.
