The electric dipole strength distribution in Sn-120 between 5 and 22 MeV has been determined at the Research Center for Nuclear Physics, Osaka, from polarization transfer observables measured in proton inelastic scattering at E-0 = 295 MeV and forward angles including 0 degrees. Combined with photoabsorption data, a highly precise electric dipole polarizability alpha(D)(Sn-120) = 8.93(36) fm(3) is extracted. The dipole polarizability as isovector observable par excellence carries direct information on nuclear symmetry energy and its density dependence. The correlation of the new value with the well-established alpha(D)(Pb-208) serves as a test of its prediction by nuclear energy density functionals. Models based on modern Skyrme interactions describe the data fairly well while most calculations based on relativistic Hamiltonians cannot.